REESE  LIBRARY 

OF  THK 

UNIVERSITY  OF  CALIFORNIA. 
No. 


ELECTRICAL  DESIGNS 


COMPRISING  INSTRUCTIONS  FOR  CONSTRUCTING 

SMALL  MOTORS,   TESTING  INSTRUMENTS 

AND  OTHER  APPARATUS;    WITH 

WORKING  DRAWINGS  FOR 

EACH   DESIGN. 


REPRINTED   FROM 

THE   AMERICAN-   ELECTRICIAN 


NEW    YORK: 

AMERICAN  ELECTRICIAN   COMPANY 
I  QO  I 


£3 


COPYRIGHT,   1901, 
BY 

AMERICAN  ELECTRICIAN  COMPANY 


PREFACE. 


The  chapters  of  this  book  originally  formed  articles  written  for  the 
AMERICAN  ELECTRICIAN  by  the  designers  of  the  apparatus  described, 
which,  in  many  cases,  had  been  actually  built  and  used  prior  to  the  pub- 
lication of  the  description.  The  designs  were  all  prepared  with  a  view 
to  reducing  to  the  simplest  degree  the  tools  and  facilities  necessary  for 
the  construction  of  the  apparatus.  The  designs  for  such  of  the  small 
motors  as  have  not  been  built  have  been  calculated  with  ample  allow- 
ance for  variations  in  the  quality  of  iron  and  steel,  so  that  any  discrep- 
ancy between  the  performance  predicted  for  these  machines  and  that 
actually  realized  will  be  in  favor  of  the  motor,  if  the  material  is  of  a  grade 
at  all  allowable  in  good  foundry  practice. 


9409? 


TABLE   OF  CONTENTS 


CHAPTER   I  PAGE 

ONE-SIXTH  HORSE-POWER  MOTOR,  WITH  DRUM  ARMATURE CECIL  P.  POOLE.  .     i 

Windings  for  ns-volt  circuit  and  battery  current. 

CHAPTER  II 
ONE-SIXTH  HORSE- POWER  MOTOR,  WITH  RING  ARMATURE, do.  ..8 

Windings  for  us-volt  circuit  and  battery  current. 

CHAPTER  III 

ONE-FOURTH  HORSE- POWER  MOTOR,  WITH  DRUM  ARMATURE,.  ...  do.  . .  15 

Windings  for  115  and  23o-volt  circuits  and  battery  current. 

CHAPTER  IV 
ONE-FOURTH  HORSE-POWER  MOTOR,  WITH  RING  ARMATURE, do.  . .  24 

Windings  for  115  and  23O-volt  circuits  and  battery  current. 

CHAPTER  V 
ONE-HALF  HORSE-POWER  MOTOR,  WITH  DRUM  ARMATURE, do.  ..  32 

Windings  for  115  and  23O-volt  circuits. 

CHAPTER  VI 

ONE  HORSE- POWER  BIPOLAR  MOTOR,  WITH  DRUM  ARMATURE do.  . .  44 

Windings  for  115  and  230- volt  circuits. 

CHAPTER  VII 

ONE  HORSE- POWER  FOUR-POLAR  MOTOR,  WITH  DRUM  ARMATURE,  do.  ..  57 

Designs  for  cast-iron  and  cast-steel  magnets  and  windings 
cor  115  and  230- volt  circuits. 

CHAPTER  VIII 

TWO  HORSE-POWER  FOUR-POLAR  MOTOR,  WITH  DRUM  ARMATURE,  do.  ..69 

Designs  for  cast-iron  and  cast-steel  field  magnets  and  wind- 
ings for  115,  230  and  soo-volt  circuits. 

CHAPTER  IX] 

THREE  HORSE-POWER  MOTOR,  WITH  DRUM  ARMATURE, do.  . .  79 

Design  for  cast-iron  field  magnet  ring  with  wrought-iron 
cores,  and  windings  for  115,  230  and  500  volts. 


VI  TABLE  OF  CONTENTS 

CHAPTER  X 

ONE-KILOWATT  COMBINED  ALTERNATING  AND  DIRECT-CURRENT 

MACHINE, J.  C.  BROCKSMITH.  .  87 

Design  for  a  one-kilowatt  machine  which  may  be  used  as  a 
direct-current  generator  or  motor;  a  single-phase,  two-phase 
or  three-phase  alternating-current  generator  or  synchronous 
motor;  a  rotary  converter  changing  single-phase,  two-phase 
or  three-phase  alternating  currents  to  direct  current;  an  in- 
verted  rotary  converter  changing  direct  current  into  single- 
phase,  two-phase  or  three-phase  alternating  currents;  a 
phase  transformer  to  effect  any  change  in  alternating  cur« 
rents  within  the  range  of  three  phases. 

CHAPTER  XI 
TWO-KILOWATT  COMBINED  ALTERNATING  AND  DIRECT-CURRENT 

MACHINE, do  ..loo 

Design  for  a  two-kilowatt  machine  which  may  be  used  as 
above. 

CHAPTER  XII 

FOUR-KILOWATT  COMBINED  ALTERNATING  AND  DIRECT-CURRENT 

MACHINE, do.  ,.107 

Design  for  a  four-kilowatt  machine  which  may  be  used  as 
above. 

CHAPTER  XIII 

SINGLE-PHASE  RECTIFIER, do.  .  .116 

A  machine  for  changing  single-phase  alternating  current 
into  direct  current. 

CHAPTER  XIV 
UNIVERSAL  ALTERNATOR  FOR  LABORATORY  PURPOSES PROF.  H.  C.  CARHART.  .125 

A  revolving-field  machine  from  which  may  be  taken  single- 
phase,  two-phase  or  three-phase  alternating  currents. 

CHAPTER  XV 
ONE-QUARTER  HORSE- POWER  SINGLE-PHASE  INDUCTION  MOTOR,.         P.  M.  HELDT.  .131 

CHAPTER  XVI 
SIMPLE  TRANSFORMER  IN  FOUR  SIZES, CECIL  P.  POOLE.  .140 

Core-type  transformer  with  a  sub-divided  primary  winding 
to  work  on  a  200,  400  or  looo-volt  circuit,  and  sub-divided 
secondary  from  which  may  be  taken  18,  32,  50  or  100  volts. 

CHAPTER  XVII 

THE  CONSTRUCTION  OF  A  REACTIVE  COIL, do.  .  .147 

A  specific  design  with  instructions  for  adapting  it  to  other 
conditions. 


TABLE  OF  CONTENTS  vii 

CHAPTER  XVIII  PAGE 

THE  CONSTRUCTION  AND  CALCULATION  OF  RHEOSTATS, P.  M.  HBLDT.  .  154 

Rules  and  formulas  governing  the  design  of  dynamo  and 
motor  rheostats. 

CHAPTER  XIX 
SIMPLE  VOLTMETERS,  AMMETERS  AND  WATTMETERS, CHAS.  T.  CHILD.  .  162 

Instructions  for  making  magnetic-vane,  permanent-magnet 
and  galvanometer-type  ammeters,  a  hot-wire  voltcaeter,  and 
dynamometer-type  and  Aron-type  wattmeters. 

CHAPTER  XX 
D'ARSONVAL  GALVANOMETER, , .Eow  pE.  SHELDON.  .174 

CHAPTER  XXI 
SENSITIVE  MIRROR  GALVANOMETER JAS.  F.  HOBART.  .  180 

CHAPTER  XXII 
THOMSON  ASTATIC  GALVANOMETER H.  S.  WEBB.  .  185 

CHAPTER  XXIII 
CHEAP  TESTING  SET, , JAS.  F.  HOBART.  .194 

CHAPTER  XXIV 
CONSTRUCTION  AND  USE  OK  A  PHOTOMETER, PROF.  A.  J.  ROWLAND.  .198 

CHAPTER  XXV 
CONSTRUCTION  OF  A  SIMPLE  STORAGE  BATTERY, CECIL  P.  POOLE.  .209 

CHAPTER  XXVI 
CONSTRUCTION  OF  A  CONSTANT- POTENTIAL  ARC  LAMP, do.  ,  .214 

CHAPTER  XXVII 
AN  EXPERIMENTAL  NERNST  LAMP, -W.  S.  FRANKLIN.  .220 

CHAPTER  XXVIII 
CONSTRUCTION  OF  AN  INDUCTION  COIL, GEO.  T.  HANCHETT.  .  223 

CHAPTER  XXIX 
CONSTRUCTION  OF  A  TESLA-THOMSON  HIGH-FREQUENCY  COIL,..  . .  A.  F.  McKissiCK.  .230 

CHAPTER  XXX 
CONDENSER  FOR  EXTREMELY  HIGH  POTENTIALS, GEO.  T.  HANCHETT.  .234 

CHAPTER  XXXI 
CONSTRUCTION  OF  A  WIMSHURST  INFLUENCE  MACHINE «...  do.  .  .237 


viii  TABLE  OF  CONTENTS 

CHAPTER  XXXII  PAGE 

I ELEPHONE  TRANSMITTER  AND  RECEIVER. E.  E.  CLEMENT.  . 243 

CHAPTER  XXXIII 
CONSTRUCTION  OF  A  DRY  BATTERY  CELL, .TOWNSEND  WOLCOTTO  =>25O 

CHAPTER  XXXIV 
SOME  HANDY  COMMUTATOR.  TOOLS, ALTON  D.  ADAMS.. 255 


CHAPTER  I. 


ONK-6IXTH  HORSE-POWER  MOTOR  WITH  DRUM  ARMATURE. 


In  preparing  this  design  and  those  which  follow,  it  has  been  assumed 
that  any  one  who  is  sufficiently  interested  in  the  subject  to  undertake  the 
construction  of  a  motor  or  dynamo  will  be  sufficiently  familiar  with 
electro-mechanics  to  exercise  individual  judgment  in  the  matter  of  fitting 
the  various  parts,  and  also  in  the  design  and  construction  of  journal  boxes, 
brush  holders,  terminal  blocks  and  such  other  parts  as  are  not  of  vital 
importance  in  the  electrical  design  of  the  machines.  Detailed  descrip- 
tions of  these  parts  will,  therefore,  not  be  given ;  the  reader  may  easily 


— ** 


FIG.    I. — ELEVATION  OF  FIELD  MAGNET. 

inform  himself  concerning  these,  if  necessary,  by  inspecting  a  finished 
machine  of  almost  any  type,  or  by  reference  to  any  good  text-book. 

The  accompanying  sketches  are  intended  to  serve  as  working  draw- 
ings in  the  construction  of  a  1-6  horse-power  motor,  for  operation  upon  a 
no-volt  direct-current  circuit.  In  Figs,  i  and  2,  M  is  the  field  magnet, 
consisting  of  a  bar  of  wrought  iron  three  inches  wide  and  one  inch  thick, 


ELECTRICAL   DESIGNS 


bent  into  the  shape  shown;  the  inner  surface  of  each  limb  is  machined 
smooth  a  distance  of  three  inches,  forming  shallow  mortises  to  receive 
the  pole-pieces,  P  P,  which  are  secured  by  Ji-in.  cap  screws  passing- 
through  the  magnet  limbs.  The  pole-pieces,  P  P,  are  of  gray  cast  iron, 
and  should  be  finished  on  all  sides  to  remove  the  scale  as  well  as  to  im- 
prove the  appearance  of  the  completed  machine.  The  magnet,  M9  might 
be  made  to  look  neater  by  touching  up  its  sides  on  a  coarse  emery  wheel ; 
it  should  be  well  annealed  after  bending  and  finishing. 

Two  holes,  h,  h,  are  bored  through  the  pole-pieces,  after  these  are 
fitted  to  the  magnet,  but  before  the  armature  chamber  is  bored  out. 
These  holes  are  17-64  in.  diameter,  and  they  must  be  3^4  ins-  apart, 
center  to  center,  and  equ'distant  from  the  center  of  the  armature  cham- 
ber; if  the  magnet  limbs  conform  strictly  to  the  measurement  given 
from  face  to  face  of  the  finished  part  of  the  limbs,  the  centers  of  the 
holes,  h,  h,  will  each  be  3-16  in,  from  the  joint  between  the  magnet 
and  the  pole  pieces.  In  these  holes  are  to  be  inserted  ^4-in.  iron  or 
steel  rods  7*4  ins-  long,  threaded  at  each  end  a  distance  of  J4  in. 


o 


o 


o 


FIG.   2. — PLAN  OF  FIELD  MAGNET  AND  JOURNAL  YOKES. 

Fig.  2,  which  gives  a  plan  view  of  the  magnet  and  the  journal  yokes, 
Y,  Y,  shows  the  function  of  these  rods ;  they  support  the  yokes  and  carry 
distance-pieces,  c,  c,  d,  dy  made  of  brass  tubing  just  large  enough  to  slip 
over  the  rods,  and  having  J/g-in.  walls.  The  pieces,  c,  c,  are  1^3  ins.  long, 
and  d,  d,  are  2%  ins.  long.  The  yokes  are  held  in  place  by  brass  nuts,  not 
shown  in  Fig.  2. 


ONE-SIXTH  HORSE-POWER  MOTOR  3 

The  journal  yokes,  Y,  Y,  are  alike.  They  are  of  cast  brass,  J/£  in. 
thick,  with  a  stiffening  rib  l/$  in.  thick,  on  each  side  of  the  journal  box. 
The  inner  end  of  one  box  should  be  trued  up  to  receive  the  brush  arm  or 
quadrant.  The  yokes  may  be  much  more  easily  and  accurately  fitted  if  a 
steel  template  is  used.  This  may  be  cheaply  provided  by  taking  a  piece 
of  flat  steel,  I  in.  wide  and  4^/2  ins.  long,  scribing  a  straight  line  approxi- 
mately down  the  center,  and  drilling  three  holes  as  shown  by  T,  Fig.  2, 
the  center  one  11-16  in.  and  the  others  ;4  in.  in  diameter.  After  the  box 
is  bored,  mount  it  on  a  mandrel  and  turn  down  the  inner  end  to  fit  the 
center  hole  in  the  template  T,  and  at  the  same  time  face  up  the  ends  of 
the  yoke  where  they  are  to  touch  the  distance-pieces ;  put  the  template  on 
the  end  of  the  box  and  scribe  the  positions  of  the  j4  m-  holes  on  the  ends 
of  the  yoke.  This  template  should  also  be  used  to  fix  the  distance  apart 
of  the  holes,  h,  h  (Fig.  i).  The  boxes  are  bored  out  9-16  in.  in  diameter 
and  fitted  with  bushings  of  J4  m-  bore  and  I  in.  long;  oil  grooves  should 
be  cut  at  each  end  of  the  box  and  provision  made  for  taking  out  the  oil. 
Oil  cups  may  be  used  to  feed  the  bearings. 

After  the  yokes  are  fitted  the  frame  may  be  centered  in  a  lathe  as  fol- 
lows, for  boring  out  the  pole-pieces.  Take  a  piece  of  J^-in.  steel  rod  it 
ins.  long,  and  make  the  shaft  ^(Fig.3)  ;  the  distance  from  e  to  g  is  3^4  ins.* 
and  the  diameter  there  is  %  in. ;  from  g  to  i  is  3  11-16  ins.  and  the  diam- 
eter y§  in. ;  from  i  to  /  is  I  1-16  ins.  and  the  diameter  J/2  in. ;  from  /  to  k  is 
3  ins.,  and  the  diameter  is  J4  m-  Turn  the  ends  of  the  shaft  down  to  a 
point,  like  that  of  a  lathe  center;  put  it  in  the  boxes,  bolt  the  yokes  in 
place,  and  then  put  the  frame  on  the  lathe  carriage,  adjusting  it  until  the 
sharp  ends  of  the  shaft  are  in  exact  line  with  the  lathe  centers.  Bolt  the 
motor  down  in  this  position,  remove  the  yokes  and  shaft,  and  bore  out 
the  pole-pieces.  The  ends  of  the  shaft  should  afterwards  be  squared  off, 
care  being  taken  to  cut  exactly  l/2  in.  off  each  end,  leaving  the  shaft  10 
ins.  long. 

The  armature  (Fig.  3)  is  built  up  of  iron  discs  3  ins.  in  diameter  and 
not  more  than  1-32  in.  thick;  there  are  twelve  slots,  each  5-16  in.  wide 
and  7-16  in.  deep.  These  may  be  punched  in  each  disc  separately,  if  a 
stamping  press  is  available,  or  they  may  be  milled  after  the  discs  are  as- 
sembled on  the  shaft.  If  the  slots  are  milled  the  discs  should  be  taken 
off  the  shaft  afterwards  and  the  burrs  dressed  off,  care  being  taken  to  re- 
assemble them  exactly  as  they  were  when  the  slots  were  milled ;  this  may 
be  accomplished  by  taking  a  very  slight  cut  with  a  metal  saw  along  the 
top  of  one  tooth,  using  the  mark  as  a  guide  to  get  the  proper  slots  to- 
gether. In  order  to  get  them  in  exact  alignment,  a  rectangular  bar  of 
metal  should  be  made  to  fit  snugly  in  one  slot  before  taking  the  discs  off; 


ELECTRICAL  DESIGNS 


when  they  are  put  back  this  bar  is  inserted  in  the  slot  to  which  it  was 
fitted  and  the  nut  is  set  up  hard.  End  plates,  WW,  of  brass,  2  ins.  in  diam- 
eter and  3-16  in.  thick,  serve  to  prevent  the  end  discs  from  buckling  when 
they  are  compressed.  A  nut  (not  shown)  fitted  to  the  thread  which  be- 
gins at  g  on  the  shaft,  serves  to  clamp  the  discs,  which  are  held  at  the 
other  end  by  the  shoulder,  i\  no  key  is  necessary  to  prevent  the  discs 
from  turning  on  the  shaft  in  so  small  a  machine,  but  it  is  essential  that 
they  should  be  clamped  as  tightly  as  a  fairly  strong  man  can  clamp  them, 
using  a  six-inch  wrench  on  the  nut.  The  shaft  may  be  held  in  a  pipe  vise 
between  i  and  /  when  setting  up  the  nut ;  the  nut  should  be  made  of  very 
hard  bronze  metal  in  preference  to  steel,  as  the  latter  attracts  magnetic 
lines  of  force  and  is  liable  to  heat. 

The  commutator  may  be  made  as  shown  in  the  sketch,  or  according 
to  any  other  modern  plan,  a  number  of  which  were  described  in  the 


1 

t                 s 

\2 

ja 

! 

3 

Am.Elec. 


FIG.    3. — DETAILS  OF  ARMATURE  AND  INSULATING  TROUGH. 


"American  Electrician"  for  July,  1896.  The  only  essential  features  are 
the  space  along  the  shaft  which  must  not  exceed  %  in.,  the  width  of  face, 
which  should  not  be  less  than  y2  in.,  and  the  number  of  segments,  which 
must  be  12.  The  commutator  here  shown  is  intended  to  be  secured  to 
the  shaft  by  a  small  steel  set-screw  through  the  hub  or  boss  at  the  front ; 
the  end  of  this  hub,  /,  must  be  1^4  ms-  from  the  end  of  the  shaft.  Ex- 
treme care  must  be  taken  to  insulate  the  segments  from  the  shell  as  well 
as  from  each  other;  mica  is  the  only  reliable  material  for  this  purpose. 
Carbon  brushes  3/2  in.  wide  and  J/J  in-  thick  should  be  used. 

The  armature  core  is  next  prepared  for  winding.    Cut  four  discs  of 
heavy  drilling  (so-called  twilled  muslin),  2^-4  ins.  in  diameter,  with  a  Y& 


ONE-SIXTH  HORSE-POWER  MOTOR 


in.  hole  in  the  center ;  varnish  the  ends  of  the  armature  core  with  shellac 
and  varnish  two  of  the  cloth  discs,  each  on  one  side ;  thread  them  on  the 
shaft,  one  at  each  end,  with  the  varnished  sides  next  to  the  core,  and  press 
them  tightly  on  the  core.  While  the  varnish  is  hardening  cut  24  pieces 
of  drilling  the  shape  of  /  (Fig.  3) ;  cut  two  slits  %  in  long  in  each  end,  7-16 
in.  from  each  side  and  5-16  in.  from  each  other;  varnish  the  strips  on  one 
side,  and  when  nearly  dry  bend  them  along  the  dotted  lines  so  as  to  form 
troughs,  with  the  varnish  inside  the  trough.  Varnish  the  outside  of  each 
trough  and  the  walls  of  the  slots  in  the  core ;  put  two  troughs  in  each  slot 
and  turn  the  flaps,  u,  v,  w,  flat  against  the  end  of  the  core,  applying 
enough  fresh  shellac  to  hold  them  down.  Then  put  on  the  two  remaining 
end  discs  of  cloth,  first  varnishing  the  sides  next  to  the  armature ;  after 
they  are  in  place  varnish  the  outsides  and  put  the  core  in  an  oven  to  bake, 
being  careful  that  the  oven  is  not  hot  enough  to  scorch  the  cloth.  A 
temperature  of  130  degs.  Fah.  is  sufficient.  After  baking,  tape  the  shaft 


-n 


FIG.   4.— MAGNET-COIL  HEAD. 


Line 

FIG.    5. — DIAGRAM  OF  STARTING  SWITCH. 


thoroughly  from  » to  /,  and  from  the  other  end  of  the  core  to  where  the 
commutator  will  come. 

The  coils  consist  of  48  turns  of  No.  24  double  cotton-covered 
wire  each,  wound  8  turns  wide  and  6  deep  in  the  slots,  but  spread  out  as 
flat  as  possible  across  the  heads.  Wind  coil  No.  I  in  slots,  A  A'  \  coil  No. 
2  in  BB'\  No.  3  in  C  C\  No.  4  in  £>£>';  No.  5  inEE',  and  No.  6  in  FF. 
Coil  No.  7  goes  in  A'  A,  on  top  of  coil  No.  i,  but  beginning  on  the  oppo- 
site side  of  the  core,  as  indicated  by  the  lettering;  No.  8  in  B'  B ;  No.  9  in 
C  C;  No.  10  in  D'  D  ;  No.  n  in  E'  E,  and  No.  12  in  F'  F.  After  winding 
each  coil  bring  the  finishing  end  across  to  the  slot  where  the  starting  end 
enters  and  twist  the  two  lightly  together.  When  all  the  coils  are  on  un- 


6  ELECTRICAL  DESIGNS 

twist  the  coil  ends  and  twist  the  last  end  of  each  coil  to  the  starting  end 
of  the  coil  in  the  slot  next  to  it  on  the  right ;  these  twisted  ends  go  each  to 
a  commutator  segment,  in  regular  order. 

The  field  magnet  is  easily  made  ready  to  wind  by  taping  the  hori- 
zontal part  of  the  magnet,  two  layers  deep,  with  varnished  muslin  and 
putting  on  two  fibre  heads.  One  of  these  heads  is  shown  by  H  (Fig.  4). 
It  is  in  two  pieces,  the  seams  being  at  the  ends,  and  is  cut  from  J^-in.  sheet 
fibre.  The  two  halves  may  be  clamped  together  on  the  core  by  means 
of  a  small  brass  wire  drawn  around  the  outer  edge,  laying  in  a  shallow 
groove,  the  ends  being  twisted  and  cut  close.  The  pole  pieces  should  be 
removed  before  taping  and  putting  on  the  heads,  to  facilitate  these  opera- 


Am.Elec. 
FIG.    6. — COMPLETED  MOTOR  FROM  THE  PULLEY  END. 

tions  as  well  as  the  winding  of  the  coil.  One  fibre  head  has  a  notch,  n, 
half  way  of  its  inner  long  side,  to  enter  the  field  wire.  The  coil  consists 
cf  No.  28  wire,  B.  &  S.  gauge,  34  layers  deep  and  170  turns  long,  making 
5,780  turns  in  all.  The  field  winding  is  connected  in  shunt  to  the  brushes, 
and  it  would  be  a  good  plan  to  provide  a  starting  switch  and  resistance 
lamp  connected  up  as  shown  diagrammatically  by  Fig.  5,  where  F  is  the 
field  coil,  B  B  the  brushes,  L  a  32-candle  power,  loo-volt  incandescent 
lamp,  5  the  starting  switch,  m  a  magnet,  and  S  W  a.  double-pole  snap 
switch.  This  arrangement  could  be  mounted  on  the  base  of  the  motor. 
Fig.  6  shows  the  complete  motor  on  a  wooden  base,  Q,  without  the  pul- 


ONE-SIXTH  HORSE-POWER  MOTOR  7 

ley ;  the  latter  may  be  any  diameter  between  ty£  ins.  and  23/2  ins.,  with 
a  i-in.  crown  face  or  J/>-in.  grooved  face.  The-  motor  is  secured  to  the 
base  by  flat  head  machine  screws  from  below,  entering  the  ends  of  the 
wrought  iron  and  countersunk  in  the  under  side  of  the  wood.  This  ma- 
chine will  stand  a  momentary  overload  of  100  per  cent.,  and  will  work  up 
to  ^4  horse-power  for  half  an  hour  at  a  time. 

WINDINGS  FOR  BATTERY  SERVICE. 

In  order  to  adapt  this  motor  for  use  in  connection  with  a  battery  the 
following  windings,  etc.,  must  be  substituted  for  those  specified  above: 
The  armature  to  be  wound  with  six  coils  of  No.  12  wire,  each  having 
twelve  turns  (three  wide  and  four  deep  in  a  slot).  The  field  wire  will 
be  No.  19,  wound  17  layers  deep  and  83  turns  in  length.  The  commu- 
tator will  have  six  segments,  and  should  have  a  brush  surface  Y^  in.  wide ; 
copper  brushes  ^  X  l/2  in.  should  be  used,  the  contact  faces  being  cut  to 
such  a  bevel  as  to  present  an  area  of  l/2  in.  square  at  least.  Connect 
the  field  winding  in  shunt  with  the  armature,  instead  of  in  series  as  is 
usually  done.  This  winding  is  for  6  volts.  The  machine  thus  wound 
will  stand  an  armature  current  of  25  to  30  amperes. 


CHAPTER  II. 


ONE-SIXTH   HORSE-POWER   MOTOR  WITH   RING  ARMATURE. 


In  Figs  7  and  8  M  is  a  wrought  iron  magnet  core,  P  P  cast-iron  pole- 
pieces,  C  the  armature  core,  and  Y  the  journal  yoke.  The  magnet  core, 
M,  is  made  from  a  %  in.  X  4j£  in.  bar  of  commercial  wrought  iron  bent 
to  the  shape  shown.  The  faces  of  the  arms  are  machined  to  a  depth  of 
1-16  in.,  where  the  pole-pieces,  P  P,  are  attached,  so  as  to  form  a  magnetic 
joint  of  as  low  reluctance  as  possible.  The  pole-pieces  are  secured  to  the 
magnet  arms  by  j4-in.  cap  screws  passing  through  smooth  holes  in  the 
arms  and  tapped  into  the  pole-pieces;  the  latter  are  of  grey  cast-iron, 
and  should  be  finished  on  all  sides  sufficiently  to  remove  the  scale.  The 
magnet,  M,  might  be  improved  in  appearance  by  touching  up  its  sides 
with  a  coarse  emery  wheel ;  it  should  be  thoroughly  annealed  after  bend- 
ing and  finishing.  It  will  be  noticed  by  reference  to  Fig.  8  that  the  ends 
of  the  magnet  arms  project  slightly  beyond  the  outer  faces  of  the  pole- 
pieces  ;  this  is  done  in  order  to  furnish  a  guide  for  the  flanges  of  the  jour- 
nal yoke  arms.  After  fitting  the  pole-pieces  to  the  magnet  arms  the  com- 
plete magnet  frame  is  bolted  to  the  lathe*  carriage  in  position  for  boring 
out  the  pole-pieces ;  before  this  is  done  it  is  necessary  to  drill  a  hole 
through  the  back  of  the  magnet  to  allow  the  boring  bar  to  pass  through, 
and  also  to  form  a  seat  for  the  rear  bearing.  This  hole  is  ^4  m-  in  diam- 
eter, and  the  magnet  frame  must  not  be  allowed  to  move  from  its  original 
position  on  the  lathe  carriage  from  the  time  the  hole  is  drilled  until  all 
the  circular  tooling  on  it  is  accomplished. 

After  drilling  the  hole  in  the  back  of  the  magnet  adjust  the  boring 
bar  and  bore  the  armature  chamber  out,  4  11-16  ins.  in  diameter.  Next 
adjust  the  boring  tool  so  that  it  will  scribe  on  the  ends  of  the  magnet  arms 
arcs  of  a  circle  6  ins.  in  diameter;  then  cut  away  the  wrought  iron  inside 
the  scribed  marks,  down  flush  with  the  pole-pieces,  as  shown  in  Fig.  7, 
forming  recesses  for  the  flanges  of  the  journal  yoke.  The  yoke  and  box 
are  cast  in  one  piece  of  brass  or  other  non-magnetic  composition ;  the 
shell  of  the  box  is  i%  ins.  long,  and  projects  l/^  in.  beyond  the  inner  face 


ONE-SIXTH  HORSE-POWER   MOTOR 


Am.  Elee. 
FIG.    7. — ELEVATION  OF  FIELD   MAGNET  AT  THE  COMMUTATOR  END. 


J 


FIG,    8. — PLAN  OF  FIELD  MAGNET  AND   DETAIL  OF  JOURNAL  BOX. 


io  ELECTRICAL   DESIGNS 

cf  the  yoke ;  the  outer  diameter  of  the  shell  is  ^4  in.,  and  it  is  bored  out  to 
7-16  in.  inner  diameter  and  bushed  to  %  m-  The  yoke  and  arm  portions 
are  3-16  in.  thick,  with  a  J/$  in.  stiffening  rib  on  each  side  of  the  box,  and 
the  arms  taper  from  i  in.  wide  at  the  flanges  to  about  y2  in.  near  the  box. 
The  flanges  are  2  ins.  long,  ^4  m-  .wide  and  %  in.  thick  after  facing;  the 
arms,  beyond  the  bends,  are  sufficiently  long  to  make  the  distance  from 
the  face  of  the  pole-piece  to  the  inner  face  of  the  yoke  2  ins.  After  boring 
the  box  it  is  mounted  on  a  stiff  mandrel  and  the  surfaces  of  the  flanges 
that  go  next  the  magnet  are  faced  up  true ;  next,  the  outer  edges  of  the 
flanges  are  skimmed  off  until  the  yoke  fits  snugly  between  the  curved 
edges  of  the  recesses  previously  cut  in  the  ends  of  the  wrought  iron  mag- 
net. Care  must  be  taken  in  making  the  pattern  for  the  yoke  that  the 
inner  edges  will  not  project  inward  beyond  the  bore  of  the  pole-pieces. 
The  yoke  is  fastened  to  the  pole-pieces  by  screws,  as  indicated  in  Fig.  7. 

The  rear  bearing,  /,  is  a  little  peculiar  in  construction.  The  box  por- 
tion is  similar  to  that  part  of  the  yoke,  but  it  is  cast  with  a  flange,  /,  I  in. 
from  the  farthest  end  of  the  shell,  which  is  1^2  ins.  long.  A  collar,  n,  is 
fitted  to  screw  onto  the  outer  end  of  the  shell,  which  is  threaded  for 
that  purpose.  The  shell  is  turned  down  outside  to  fit  snugly  in  the  hole 
drilled  in  the  back  of  the  magnet,  and  when  it  is  inserted  in  the  hole  the 
collar,  n,  is  put  on  and  screwed  up  tight.  This  box,  like  the  front  one, 
is  bushed  to  j4  in-  bore.  The  drawing  shows  the  flange,  /,  and  collar,  M, 
countersunk  in  the  metal  of  the  magnet ;  this  will  not  be  necessary  if  the 
magnet  is  smoothed  up  with  an  emery  wheel,  as  above  suggested,  the 
object  in  countersinking  being  to  provide  smooth,  true  bearing  surfaces 
for  the  flange  and  collar. 

The  armature  core,  spider  and  shaft  are  shown,  partly  in  cross-sec- 
tion, by  Fig.  9.  The  core  is  built  up  of  charcoal  iron  (not  steel)  rings, 
4*/2  ins.  outside  diameter  and  2l/2  ins.  inside,  not  more  than  1-32  in.  thick ; 
these  are  assembled  on  a  brass  drum,  shown  by  Fig.  n,  which  should  be 
25/&  ins.  outside  diameter  before  finishing,  so  that  it  may  be  turned  down 
to  exactly  fit  the  inner  circle  of  the  armature  rings ;  the  wall  of  the  drum 
is  y%  in.  thick  after  finishing,  and  there  are  four  equidistant  projecting 
lugs,  /,  }/2  in.  long,  on  each  end  by  which  the  drum  is  secured  to  the  spi- 
der (see  Figs.  9  and  io).  The  rings  forming  the  core,  C  (Fig.  9),  are 
compressed  and  held  on  the  drum,  r,  by  two  brass  washers,  w,  w,  3-16  in. 
thick  and  3^  ins.  outer  diameter,  which  screw  onto  the  ends  of  the  drum. 
The  core,  when  compressed,  is  ift  ins.  long,  and  has  20  slots  J4  in-  wide 
and  7-16  in.  deep;  the  washers,  w,  w,  must  be  set  up  as  tight  as  the 
threads  will  stand. 

The  spider,  s  (Figs.  9  and  io),  is  made  of  brass,  and  consists  of  a  hub 


ONE-SIXTH  HORSE-POWER   MOTOR 


II 


in.  diameter,  2  ins.  long  and  3/2 


in.  bore)  and  four  arms  having  T- 
shaped  ends,  the  wide  part  or  heads  of  which  project  beyond  the  arms 
and  hub  at  each'  end,  the  length  of  these  heads  being  2j^  ins.  and  their 
width  y§  ins.  The  heads  of  the  spider  arms  are  turned  off  to  fit  very 
closely  inside  the  drum,  r,  which  is  mounted  on  the  spider  in  such  a  posi- 
tion as  to  bring  the  spider  arms  in  alignment  with  the  lugs,  /,  of  the  drum; 


FIG.    9. — AXIAL  SECTION   OF  ARMATURE 
"  *•  CORE. 


FIG.    10. — END   OF   ARMATURE   CORE. 


An.  Elec. 
FIG.    II. — ARMATURE   CORE  DRUM. 


screws  through  the  spider  arms  into  the  lugs  hold  the  drum  and  spider 
together. 

The  shaft,  S,  is  &l/2  ins.  long;  the  portion  /is  ij/s  ins.  long  and  *4  m- 
diameter;  k  is  i  in.  long  and  ^8  in.  diameter;  the  part  passing  through 
the  core  is  3  ins.  long  and  J/2  in.  in  diameter ;  m  is  ^  in.  long  and  y%  in. 
diameter ;  and  p  is  3  in.  long  and  *4  in.  diameter.  The  spider,  s,  may  be 
secured  to  the  shaft  by  a  key  or  a  set-screw ;  the  set-screw  is  sufficient  in 


12  ELECTRICAL  DESIGNS 

so  small  a  machine.  The  commutator  (not  shown)  must  not  be  more  than 
24  in-  over  all,  along  the  shaft ;  it  must  have  y2  in.  brush  surface  and  20 
segments ;  other  details  may  be  made  to  suit  the  will  of  the  builder.  The 
front  end  of  the  commutator  must  be  not  less  than  3-16  in.  from  the 
shoulder  where  /  and  k  join. 

The  armature  is  next  prepared  for  winding  by  removing  the  drum 
and  core  from  the  spider  and  insulating  the  ends  and  interior  of  the  core 
and  the  walls  of  the  slots.  Cut  four  rings  of  heavy  drilling  of  a  size  to  cover 
the  washers,  w  w,  and  the  ends  of  the  drum,  r ;  varnish  two  of  them  on  one 
side  with  shellac,  and  apply  them  to  the  ends  of  the  armature  body. 
While  these  are  hardening  cut  forty  strips  of  drilling  ij£  ins.  wide  and 
2^4  ins.  long;  in  each  end  of  each  of  these  cut  two  slits  54  m-  l°ng  parallel 
with  the  sides,  and  located  7-16  in.  from  each  side  of  the  strip.  Varnish 
these  on  one  side,  and  when  nearly  dry  fold  them  into  troughs  to  fit  the 
slots,  two  troughs  to  a  slot,  one  within  the  other;  fold  them  so  that  the 
varnish  will  be  on  the  inside  of  the  trough. 

When  these  are  dry  varnish  the  slots  and  the  outsides  of  the  troughs 
and  put  the  latter  in  the  slots,  bending  the  ends  flat  against  the  core  and 
securing  them  there  with  a  little  fresh  varnish.  Then  varnish  the  ends 
of  the  core  (two  cloth  rings  being  on  them),  and  one  side  of  the  two  re- 
maining rings  of  drilling ;  put  these  rings  on  top  of  the  first  ones,  varnish 
them  on  the  outside,  and  put  the  core  in  an  oven  to  bake.  The  armature 
coils  consist  of  No.  24  double  cotton-covered  wire,  wound  six  turns  wide 
and  twelve  layers  deep.  Before  winding  them  four  strips  of  wood  3  ins. 
long.  y%  in.  wide  and  ^  in.  thick  should  be  screwed  to  the  inner  wall  of 
the  brass  drum,  in  line  with  the  lugs,  /,  so  as  to  preserve  the  spaces  for  the 
four  arms  of  the  spider.  A  double  thickness  of  drilling  should  also  be 
applied  to  the  interior  of  the  drum  to  insulate  the  coils  from  it.  The 
connections  are  the  simple  Gramme  ring  arrangement. 

The  field  winding  is  necessarily  divided  into  two  coils,  on  account  of 
the  rear  bearing  passing  through  the  magnet.  Kach  coil  consists  of  No. 
28  double  cotton-covered  wire,  wound  17  layers  deep  and  181  turns  or 
more  long;  the  two  coils  are  connected  in  series  with  each  other  and  in 
shunt  to  the  brushes.  Heads  of  hard  fibre  J/£  in.  thick  should  be  used  to 
protect  the  ends  of  each  coil ;  one  of  these  is  shown  by  H  (Fig.  12),  but  the. 
width  should  be  7-16  in.  instead  cf  ^4  in.  as  marked.  11 

It  is  in  two  pieces,  the  seams  being  at  the  ends,  and  is  cut  from  y%  in.  ' 
sheet  fibre.    The  two  halves  may  be  clamped  together  on  the  core  by 
means  of  a  small  brass  wire  drawn  around  the  outer  edge,  laying  in  a 
shallow  groove,  the  ends  being  twisted  and  cut  close.     The  pole-pieces 
should  be  removed  before  taping  and  putting  on  the  heads,  to  facilitate 


OXE-SIXTH  HORSE-POWER  MOTOR  13 

these  operations  as  well  as  the  winding  of  the  coil.  One  fibre  head  has  a 
notch,  n,  half  way  of  its  inner  long  side  to  enter  the  field  wire.  The  pole- 
pieces  should,  of  course,  be  removed  before  winding  the  field  coil,  and 
the  magnet  core  should  be  wrapped  with  two  layers  of  varnished  drilling 
where  the  coils  are  to  go.  The  entering  end  of  each  coil  should  be  re- 
mote from  the  journal,  and  this  means  that  the  magnet  must  be  turned 
end  for  end  after  one  coil  is  wound,  or  else  the  two  coils  must  be  wound 
in  opposite  directions  in  order  that  the  free  ends  at  the  center  of  the  mag- 
net may  be  connected  together.  It  is  advisable  to  provide  a  starting 
switch  similar  to  the  one  shown  diagrammatically  by  Fig.  13,  where  F. 
is  the  field  coil;  J3  B  the  brushes;  S  the  starting  switch  lever:  L  a  3 2 -can- 
dle-power no-volt  lamp;  M  a  magnet,  and  SW  a  double-pole  snap 
switch. 

The  motor  is  intended  to  be  mounted  on  a  wooden  base-board  8  ins. 
X  8  ins.,  a  cleat  3  ins.  wide  and  7-16  in.  thick  being  put  under  the  pole- 


H 


Am.Elec. 
FIG.    12. — MAGNET  COIL  HEAD. 


.Cine 


FIG.    13. — DIAGRAM   OF  STARTING  SWITCH. 


pieces  so  as  to  clear  the  field  coil.  Bolts  from  beneath,  tapped  into  the 
magnet  and  countersunk  in  the  under  side  of  the  base,  should  be  used  to 
hold  the  motor  on  the  base.  The  pulley  may  be  any  diameter  from  i  in. 
to  3  ins.  by  I  in.  face,  if  crowned,  or  }4  in.  if  grooved. 


WINDINGS  FOR  BATTERY  CURRENT. 


The  armature  will  be  wound  with  10  coils  of  No.  12  wire,  each  coil 
having  16  turns  and  occupying  two  (adjacent)  slots.  The  field  wire  will 
be  No.  1 8,  wound  8  layers  deep  and  40  turns  long  in  each  coil;  the  two 
coils  containing  640  turns  in  all.  The  commutator  must  have  10  segments 


14  ELECTRICAL   DESIGNS 

and  a  brush  surface  $£  in.  wide;  copper  brushes  ^  X  /4  in.  should  be 
used,  the  contact  faces  being  cut  to  such  a  bevel  as  to  present  a  surface 
at  least  }4  in.  square  each.  The  field  winding  is  to  be  connected  in  shunt 
to  the  brushes,  instead  of  in  series  as  is  usually  the  practice  in  battery 
motor  construction.  This  winding  is  for  6  volts  at  the  terminals;  the 
current  required  will  depend  upon  the  work  done;  the  machine  is  cap- 
able of  standing  an  armature  current  of  25  to  30  amperes. 


CHAPTER  III. 


ONE-FOURTH  HORSE-POWER  MOTOR  WITH  DRUM  ARMATURE. 


Fig.  14  represents  the  field  magnet,  and  Fig.  15  one  of  the  journal 
yokes.  The  magnet  is  of  the  familiar  single-coil  type  employed  by  West- 
inghouse,  Jenney  and  others.  The  core  is  of  round  Norway  iron,  2  ins, 
in  diameter  and  9  ins.  long  over  all.  The  ends  are  turned  tapering,  as  in- 
dicated by  dotted  lines,  to  insure  intimate  contact  with  the  yokes ;  the 
taper  is  from  the  full  diameter  to  1^4  ins.,  and  begins  2  ins.  from  each  end. 
The  pole-pieces  are  of  cast-iron.  Fig.  16  gives  a  plan  view  and  a  face 
view  of  one  pole-piece,  from  which  all  the  essential  dimensions  may  be 
obtained.  The  arms  which  support  the  journal  yokes  are  cast  solid  with 
the  pole-pieces,  and  their  horizontal  thickness  tapers  from  }A  in.  at  the 
pole-piece  to  y\  in.  where  the  yoke  is  bolted  on. 

In  fitting  the  magnet  frame  together  the  best  procedure  is  to  bore  the 
tapered  holes  in  the  lower  part  of  each  pole-piece  and  turn  the  ends  of  the 
magnet  core  to  the  same  taper,  but  just  a  trifle  large ;  then  dress  each 
taper  down  very  gradually  with  a  fine  file  (the  core  being  run  in  a  lathe) 
until  the  pole-piece  can  be  pushed  on  by  hand  far  enough  to  bring  the  end 
of  the  core  within  1-32  in.  of  the  back  surface  of  the  cast-iron.  The  pole- 
pieces  and  ends  of  the  core  should  be  punch-marked,  so  as  to  insure  finally 
mounting  each  pole-piece  on  the  end  to  which  it  was  fitted.  After  dress- 
ing down  the  ends  of  the  core  as  above  described,  drill  and  tap  in  each 
end  a  hole  for  a  X~m-  machine  screw,  the  purpose  of  which  will  be  ap- 
parent by  glancing  at  the  right-hand  end  of  the  magnet  in  Fig.  14,  where 
C  is  a  four-armed  claw  or  spider  with  a  hole  through  the  center  where 
the  arms  intersect.  The  arms  are  3-16  in.  thick,  measured  at  right  angles 
to  the  bolt,  and  taper  from  3-16  in.  to  }$  in.  thick  measured  parallel  with 
it.  One  of  these  spiders  is  used  at  each  end,  though  the  drawing  shows 
it  at  only  one  end  of  the  machine. 

After  drawing  one  pole-piece  home  solid  by  means  of  its  spider  and 
bolt,  slip  the  other  pole-piece  on  loosely  and  clamp  the  pole-pieces  lightly 


i6 


ELECTRICAL  DESIGNS 


FIG.    15.— PLAN  OF  JOURNAL  YOKE. 


dxierfcgi)  Electrician. 
FIG.    14. — ELEVATION  OF  FIELD  MAGNET  WITH  JOURNAL  YOKE  IN   POSITION. 


ONE-FOURTH  HORSE-POWER  MOTOR 


FIG.    l6. — PLAN  AND  FACE  VIEWS  OF  ONE  FIELD-MAGNET  POLE-PIECE. 


i8 


ELECTRICAL  DESIGNS 


between  two  iron  plates  with  planed  surfaces,  applied  between  the  journal 
arms,  so  as  to  keep  the  four  horns  of  the  pole-pieces  in  alignment ;  then 
force  the  second  pole-piece  home  by  means  of  its  bolt  and  spider,  and 
clamp  the  horns  hard  between  the  iron  plates.  The  bottom  surfaces  of 
the  cast-iron  pieces  should  then  be  trued  up  on  a  planer  or  shaper  and  the 
clamps  taken  off  the  pole-piece  horns. 

The  next  operation  is  boring  the  armature  chamber  and  the  seats 
for  the  journal  yokes.  The  armature  chamber  bore  is  4  3-16  ins;  the 
seats  for  the  journal  yokes,  marked  "finished  part"  in  Fig.  16,  are  bored 
or  cut  to  4^  ins.  diameter,  and  this  must  be  done  before  the  position  oi 
the  machine  is  disturbed  after  boring  the  armature  chamber.  This  com- 
pletes the  machine  work  on  the  magnet,  except  the  bolt  holes. 

The  journal  yoke  may  be  made  of  brass  or  any  composition  metal. 
The  bar  is  3-16  in.  thick  and  i  in.  wide,  except  near  the  ends,  where  it 


FIG.    17. — SHAFT  AND   CROSS    SECTION  OF  ARMATURE  CORE. 

flares  to  correspond  with  the  width  of  the  arms.  At  each  end  is  a  right- 
angled  lug,  y§  in.  thick  after  machining;  these  lugs  fit  the  seats  in  the 
ends  of  the  iron  arms,  and  the  yokes  should  be  fitted  to  the  magnet  imme- 
diately after  finishing  the  machine  work  on  the  latter,  and  before  it  is 
taken  apart  to  put  on  the  coil.  The  box  portion  is  i]/2  ins.  long  over  all, 
3-16  in.  of  its  length  being  on  the  inside  of  the  yoke  and  iJ/6  ins.  on  the 
outside.  As  shown  by  the  plan  view  of  the  yoke  in  Fig.  15,  there  are 
stiffening  webs  starting  flush  near  the  ends  of  the  yoke  and  attaining  a 
width  of  24  m-  at  the  box;  these  are  j£  in.  thick.  The  box  is  j£  in.  in 
outer  diameter,  and  bored  to  17-32  in.  inside;  it  is  bushed  to  ^  in.  diam- 
eter. These  latter  dimensions,  excepting  the  final  inside  diameter  of  the 


ONE-FOURTH  HORSE-POWER  MOTOR 


bushing,  may  be  varied  to  suit  individual  ideas,  as  may  also  the  design 
of  the  box.  The  only  essential  measurements  are  those  of  the  yoke-bar, 
the  length  of  the  box  and  the  bore  of  the  journal  bushing.  The  journal 
yokes  are  held  in  place  by  l/±  in.  cap  screws  passing  through  the  iron  arms 
and  tapping  into  the  lugs  of  the  yokes.  . 

Figs.  17,  1 8  and  19  show  the  shaft  and  armature  core  (the  latter  in 
cross-section),  an  armature  disc,  and  the  shell  and  head.  The  discs  are  of 
charcoal  iron,  4  ins.  outside  diameter  with  a  iJ/^-in.  hole  in  the  center  and 
a  ^s-in.  key-seat,  annealed  after  punching  and  key-seating;  there  are 
eighteen  slots  3/£  in  wide  and  3/£  in.  deep.  The  shell  and  one  head  are 
cast  in  one  piece  (of  brass),  and  consist  of  a  barrel  ij4  ms.  outside  diam- 
eter (when  finished)  and  2  ins.  long,  with  a  head,  s,  at  one  end,  3^  ins.  in 
diameter  and  tapered  in  thickness  from  3/4  in.  near  the  center  to  1-16  in. 
at  the  periphery;  at  the  opposite  end  of  the  barrel  is  a  cross-bar  J/&  in. 
thick,  cast  with  the  barrel  and  cf  the  shape,  shown,  being  3/£  in.  wide  where 
it  joins  the  barrel  and  Y\  in.  at  the 
center.  A  j^j-in.  hole  is  drilled  in 
the  center  of  this  cross-bar  and  an- 
other in  the  center  of  the  head,  s,  at 
the  other  end  of  the  barrel;  the  shell 
is  mounted  on  a  mandrel,  the  barrel 
turned  down  to  fit  the  hole  in  the 
armature  discs,  and  both  sides  of  the 
head  faced  off  smooth.  A  ^6-in  key- 
scat  3-16  ins.  deep  is  cut  in  the  bar- 
rel so  as  to  come  in  the  center  of  one 
end  of  the  cross-bar, as  shown;  a  ^j-in. 
X  /^-in.  feather,  or  parallel  key,  is 
laid  in  the  key-seat,  and  the  discs 
threaded  on  the  barrel  and  compressed 
against  the  head  by  the  collar,  /i, 

and  two  bolts  (not  shown)  passing  through  the  collar  and  inside  the  bar- 
rel, and  tapping  into  the  head  at  the  other  end.  This  collar,  h,  is  of  brass, 
3^  ins.  in  diameter  and  tapering  from  3-16  to  1-16  in.  in  thickness  when 
finished.  The  opening  in  the  center  should  fit  the  outline  of  the  cross-bar 
en  the  end  cf  the  barrel  at  least  closely  enough  to  prevent  the  collar  from 
shifting  under  stress  of  centrifugal  force ;  the  collar  must  be  finished  up 
smooth  on  both  sides.  A  disc  of  insulation  should  be  put  on  next  to  the 
brass  head  before  the  iron  discs  are  put  on,  and  another  insulating  disc 
should  go  between  the  last  iron  disc  and  the  clamping  collar,  h. 

If  the  slots  are  cut  in  the  core  with  a  milling  machine  the  discs  must 

•^^^ 


American  Electrician] 
FIG.    l3. — AN  ARMATURE   DISC. 


20  ELECTRICAL  DE^IUN'S 

all  come  off  the  barrel  to  have  the  burrs  removed,  and  also  be  reannealed ; 
the  key-seat  will  insure  their  returning  in  the  original  angular  position. 
It  is  much  better  to  have  discs  with  the  slots  punched  before  the  first  an- 
nealing. The  shaft  is  lofy  ms-  long  over  all ;  J/2  in.  in  diameter  in  the 
largest  part,  7-16  in.  where  the  commutator  goes  and  ^  in.  in  the  jour- 
nals. A  i-i6-in  X  /^-in.  collar,  c,  is  shown  back  of  the  armature,  the  pur- 
pose of  which  is  merely  to  "locate"  the  armature  shell ;  it  is  not  absolutely 
necessary,  however,  and  may  be  left  off  if  desired.  The  easiest  way  to 
provide  for  it  is  to  make  the  shaft  of  Y&-m.  stock,  leaving  the  original 
metal  to  form  the  collar  when  turning  the  shaft  to  proper  diameter.  The 
armature  shell  may  be  keyed  to  the  shaft  or  pinned  obliquely  through  the 
thick  part  of  the  head;  it  must  be  positively  secured  by  some  such  means. 
The  commutator  shell  must  be  bored  to  fit  the  ^-j6-in.  portion  of  the 
shaft,  and  must  not  exceed  1*4  ins.  along  the  shaft.  The  lugs  where  the 
wires  are  attached  to  the  segments  may  project  toward  the  armature  % 


American  Electrician 
FIG.    IQ. — ARMATURE  CORE   DRUM  AND  HEAD. 

in.  or  so.  There  must  be  eighteen  segments,  and  a  diameter  of  2  ins.  is 
recommended.  The  quadrant  carrying  the  brush-holders  should  be  fitted 
to  the  inner  end  of  the  journal  box,  and  carbon  brushes  not  smaller  than 
J4  in.  X  /2  in.  (one  on  each  side)  on  the  contact  surface  should  be  used. 
If  the  machine  be  used  as  a  dynamo  (it  will  maintain  five  or  six  no-volt 
lamps)  metal  brushes  of  the  same  surface  should  be  used  to  reduce  the  re- 
sistance of  the  brush  contact. 

The  field  coil  contains  37  layers  of  No.  28  double  cotton-covered 
wire.  After  the  magnet  is  fitted  as  described  in  the  beginning  of  the 
article  it  is  taken  apart  and  two  circular  magnet  heads  of  fibre  ^  in. 
thick  and  3J4  m-  outer  diameter  are  put  on  with  a  driving  fit,  care 
being  taken  that  the  distance  along  the  core  from  outside  to  outside 
of  the  heads  corresponds  with  the  distance  between  the  pole-pieces  when 


ONE-FOURTH  HORSE-POWER  MOTOR  21 

the  whole  is  assembled.  A  groove  must  be  cut  on  the  inner  face  of 
one  head  from  the  center  to  the  outer  edge  in  order  to  lead  out  the 
starting  end  of  the  field  wire,  and  this  must  be  covered  with  two 
layers  of  oil  paper  to  prevent  short-circuiting  the  successive  layers  of  the 
coil.  The  core  must  be  insulated  with  three  layers  of  shellaced  muslin 
between  the  heads  and  the  field  wire  put  on  evenly,  care  being  taken  not 
to  "spread"  the  heads  ;  if  the  winding  is  carefully  done  the  coil  will  be  216 
turns  in  length.  The  number  of  turns  in  length  is  not  a  vital  matter,  but 
the  depth  must  be  37  layers.  The  ampere  turns  are  the  same  no  matter 
what  the  length  of  the  coil,  but  it  should  be  as  long  as  possible  in  order  to 
reduce  the  heat  loss. 

After  winding  the  coil  and  securing  the  ends  one  pole-piece  is  put  on 
solid  and  the  other  one  slipped  on  until  it  begins  to  bind,  when  the  journal 
yokes  must  be  inserted  between  their  arms  and  the  bolts  put  in  as  far  as 
possible  without  jamming.  Then  by  tightening  up  the  journal  yoke  bolts 
and  the  pole-piece  bolt  together,  being  particular  never  to  draw  the  yoke 
bolt  hard  against  the  arm,  the  frame  will  come  together  in  its  original  po- 
sition. As  an  additional  precaution  it  may  be  set  on  a  true  plane  surface, 
and  if  the  base  of  the  loose  pole-piece  gets  out  of  alignment  tap  the  horn 
lightly  until  the  frame  is  true  on  the  bottom.  The  magnet  frame  must 
be  provided  with  a  non-magnetic  base ;  hard  wood  is  as  good  as  anything, 
the  frame  being  secured  by  flat-head  brass  machine  screws  from  below, 
two  in  each  casting,  countersunk  in  the  wood. 

The  armature  winding  is  divided  into  eighteen  coils,  each  having  45 
turns  of  No.  22  double  cotton-covered  wire,  9  turns  wide  and  5  turns  deep 
in  the  slot.  The  slots  must  be  insulated  with  troughs  of  muslin  and 
mica,  or  preferably  flexible  micanite,  0.03  in.  thick.  The  troughs  are 
easily  made  by  cutting  the  material  into  strips  2l/2  ins.  long  by  i*/&  ins. 
wide,  and  slitting  the  ends  so  as  to  permit  the  projecting  portion  of  the 
trough  to  be  folded  back  flat  against  the  core.  Before  putting  in  the 
troughs  a  disc  of  heavy  drilling  3^4  ins.  in  diameter  should  be  secured 
to  each  end  of  the  core  by  means  of  varnish,  and  the  outer  faces  varnished 
and  allowed  to  nearly  dry.  Then  put  in  the  troughs  and  put  on  two  more 
muslin  discs,  varnishing  the  whole,  and  bake  until  thoroughly  dry.  In- 
stead of  winding  each  coil  in  diametrically  opposite  slots,  take  slots  lack- 
ing one  of  being  precisely  opposite. 

A  good  plan  is  to  make  a  sketch  of  an  armature  disc  and  number  the 
slots  from  left  to  right  successively  around  the  periphery.  Then  wind 
the  coils  as  follows,  the  coil  numbers  indicating  the  order  in  which  the 
coils  are  put  on,  not  the  order  in  which  they  are  connected  to  the  com- 
mutator. 


22  ELECTRICAL  DESIGNS 

COIL  NO. — i     2      34      5      6      7      8      9   10    n   12   13   14   15    16   17   is 

STARTS  IN  SLOT  NO.--I  IO   13   4   7   l6   2  II   1$   6   14   5  l8   9   3  12   8  17 
ENDS  IN  SLOT  NO. — 9  l8    3  12   15    6  IO   I    5  14    5  13   8  17  II   2  l6  17 

Each  pair  of  coils  must  be  covered  with  muslin  where  they  cross  the 
heads  before  the  next  pair  is  put  on,  and  before  coil  No.  8  is  wound  on  top 
of  coil  No.  I  in  slot  No.  I  the  bottom  coil  must  be  insulated  by  a  strip  of 
micanite  laid  in  the  slot ;  this  is  true  of  every  bottom  coil. 

After  the  winding  is  on,  and  before  connecting  up  to  the  commuta- 
tor, the  band  wires  should  be  put  on.  Use  No.  19  B.  W.  G.  soft  tinned- 
iron  wire,  known  by  hardware  dealers  as  "white  stove-pipe  wire,"  for  the 
bands,  and  put  them  on  under  as  heavy  pressure  as  possible  without  en- 
dangering the  armature  shaft.  Two  bands  of  eight  turns  each,  y2  in.  from 
each  end  of  the  core,  will  suffice.  A  strip  of  mica  between  two  strips  of 
fullerboard  must  go  under  each  band,  and  the  bands  should  be  soldered 
at  intervals,  not  all  the  way  around.  Four  tin  clips  located  equidistantly, 
with  a  dab  of  solder  at  each,  will  give  ample  security. 

The  technical  data  for  this  machine  are  as  follows : 

TERMINAL  E.    M.    F. ,    IIO  VOLTS. 

Armature  current,  normal 1.9  amps. 

"  "          maximum 2.3       " 

"        resistance,  warm 3.33  ohms 

Field  current  at  no  volts . . . .     .25  amp. 

"      resistance,  warm 440  ohms 

C7a^?loss  in  field 27^  watts 

C*R  loss  in  armature 12-}-     " 

Hysteresis  loss  in  armature 20 —     " 

Magnetic  flux  per  square  inch  : 

In  field  core 90,000  lines 

In  pole-pieces 39,000     " 

In  air-gap 25,250     " 

In  armature  teeth 68,000     " 

In  armature  core 56,000     " 

Co-efficient  of  leakage 1.4 

Electrical  efficiency 84  percent. 

Commercial  efficiency  (friction  10  p.  c.  estimated). ...          65         f * 
Revolutions  per  minute 2,ooo 

If  it  is  desired  to  build  a  smooth-core  machine  the  armature  core 
must  be  made  3^  ins.  in  diameter,  and  two  grooves  y$  in.  wide  and  5-16 
in.  deep  must  be  cut  in  the  face  of  the  core  at  opposite  points  for  the  re- 
ception of  driving  teeth.  These  are  two  pieces  of  fibre  %  in.  thick,  $4  in. 
wide  and  2  ins.  long,  set  on  edge  in  the  grooves,  and  projecting  3-16  in. 
above  the  surface  of  the  core.  The  core  must  be  thoroughly  covered  with 


ONE-FOURTH  HORSE-POWER  MOTOR  23 

two  layers  of  micanite  cloth.  The  number  of  coils  is  the  same  as  before, 
"but  the  coils  will  be  18  turns  wide  and  5  deep;  and  in  this  case  they  are 
net  superposed,  the  depth  of  a  coil  (5  layers)  being-  the  total  depth  of  the 
winding.  The  guiding-  diagram,  therefore,  must  divide  the  periphery  of 
the  armature  into  36  spaces  instead  of  18,  because  each  space  now  con- 
tains one  side  of  only  one  coil.  The  smoothest  winding  will  be  as 
follows : 

COIL  NO. —  I   23  4  5   6   7  8  9  10  ii  12  13  14  15  16  17  18 

STARTS  IN  SPACE  NO.—  I  ig   J  2$  3!  13   3  21  27   Q  33  15  23   5  II   2Q  35   17 
ENDS  IN  SPACE  NO. — 18  36  24   6  12  50  2O   2   8  26  14  32   4  22  28   IO  l6  34 

Care  must  be  taken  in  connecting-  up  either  of  the  armature  windings 
to  take  the  starting  ends  cf  the  coils  in  proper  succession  to  the  commu- 
tator segments ;  the  outer  end  of  each  coil  goes  to  the  segment  on  the 
ri  jht  of  the  one  to  which  the  starting  end  is  led.  The  smooth-core  arma- 
ture is  banded  just  as  the  slotted  one  is,  except  that  soft  brass  wire 
must  be  used  instead  of  tinned  iron. 


CHAPTER  IV. 


ONE-FOURTH  HORSE-POWER  MOTOR  WITH  RING  ARMATURE. 


This  machine  has  a  field  magnet  of  exactly  the  same  design  as  the 
one  last  described.,  the  only  difference  being  in  the  dimensions.  The 
instructions  for  fitting  up  the  magnet  shown  by  Figs.  14  and  15,  therefore, 
apply  to  this  one.  The  size  of  the  magnet  core  and  yokes  shown  by  Fig. 
14  also  apply  to  this  magnet.  Figs.  21  and  22  give  all  of  the  dimensions 
for  this  magnet  frame  that  differ  from  those  of  the  previous  one,  excepting 
the  bore  of  the  armature  chamber,  which  is  5  3-16  ins.  instead  of  4  3-16 
ins.  The  lugs  that  support  the  journal  yokes  are  set  one  inch  wider  apart 
than  in  the  drum  armature  motor,  and  the  seats  for  the  ends  of  the  journal 
3'okes  are  bored  or  cut  to  5^  ins.  diameter.  As  in  the  former  case,  this 
boring  must  be  done  before  the  frame  is  moved  from  the  position  it  oc- 
cupied during  the  boring  of  the  armature  chamber. 

The  journal  yokes  may  be  made  of  anything  except  iron  and  steel. 
The  bar  is  3-16  in.  thick  and  I  in.  wide.,  except  near  the  ends,  where  it 
flares  to  correspond  with  the  width  of  the  arms.  At  each  end  is  a  right- 
angled  lug,  *X>  in.  thick  after  machining;  these  lugs  fit  the  seats  in  the 
ends  of  the  iron  arms,  and  the  yokes  should  be  fitted  to. the  magnet  im- 
mediately after  finishing  the  machine  work  on  the  latter,  and  before  it  is 
taken  apart  to  put  on  the  coil.  The  box  portion  is  \y2  ins.  long  over  all, 
3-16  ins.  of  its  length  being  on  the  inside  of  the  yoke  and  i  J^  ins.  on  the 
outside.  As  shown  by  the  plan  view  of  the  yoke,  Fig.  22,  there  are  stiff- 
ening webs  starting  flush 'near  the  ends  of  the  yoke  and  attaining  a  width 
of  £4  in,  at  the  box;  these  are  %  in  thick.  The  box  is  j£  in.  in  outer  di- 
ameter, and  bored  to  17-32  in.  inside;  it  is  bushed  to  J6  in.  diameter. 
Most  of  the  dimensions  of  the  yoke  and  box  may  be  varied  to  suit  individ- 
ual ideas,  as  may  also  the  design  of  the  box.  The  only  essential  measure- 
ments are  the  length  of  the  yoke-bar,  the  length  of  the  box  and  the  bore 


ONE-FOURTH  HORSE-POWER  MOTOR 


FIG.  22. — PLAN  OF  JOURNAL  YOKE. 


American  Electrician 
FIG.    20. — ELEVATION  OF  FIELD  MAGNET  WITH  JOURNAL  YOKE  IN   POSITION. 


ELECTRICAL  DESIGNS 


J 

s 


Finishcd 


Finished 


FIG.    21. — PLAN  AND  FACE  VIEWS   OF  ONE  FIELD-MAGNET  POLE-riECE. 


ONE-FOURTH   HORSE-POWER   MOTOR 


27 


cf  the  journal  bulling.  The  journal  yokes  are  held  in  place  by  *4  m- 
cap-screws  passing  through  the  iron  arms  and  tapping  into  the  lugs  of 
the  yokes.  )  .  . 

The  armature  core,  spider  and  shaft  are  shown,  partly  in  cross-sec- 
tion, by  Figs.  23  and  24.  The  core  is  built  up  of  charcoal  iron  (not  steel) 
discs  5  ins.  outside  diameter  and  2^  ins.  inside,  not  more  t/.an  1-32  in. 
thick ;  these  are  assembled  on  a  brass  drum  i^  ins.  long  (Fig.  25).  which 


FIG.  23. — AP"VTURE  CORE  AND  SHAFT. 


FIG.  24. — END  OF  ARMATURE  CORE. 


FIG.  25. — ARMATURE  CORE  DRUM, 


should  be  2^4  ms.  outside  diameter  before  finishing,  so  that  it  may  be 
turned  down  to  exactly  fit  the  inner  circle  of  the  armature  rings;  the 
wall  of  the  drum  is  y%  in.  thick  after  finishing,  and  there  are  four  equi- 
distant projecting  lugs,  /,,  ^3  in.  wide  and  l/2  in.  long,  on  each  end,  by 
which  the  drum  is  secured  to  the  spider  (see  Figs.  22  and  23).  The  rings 
forming  the  core,  C  (Fig.  22),  are  compressed  and  held  on  the  drum,  r, 
by  two  brass  washers,  w,  w,  3-16  in.  thick  and  3^  ins.  outer  diameter, 


28  ELECTRICAL  DESIGNS 

which  screw  onto  the  lugs  and  ends  of  the  drum.  The  core  when  com- 
pressed is  1^2  ins.  long,  and  has  20  slots  3-10  in.  wide  and  */2  in.  deep; 
the  washers,  w  w,  must  be  set  up  as. tight  as  the  threads  will  stand. 

The  spider,  s  (Figs.  23  and  24),  is  made  of  brass,  and  consists  of  a  hub 
(%  in.  in  diameter,  2r/2  ins.  long  and  $/$  in.  bore)  and  four  arms  having 
T-shaped  ends,  the  wide  part  or  heads  of  which  project  beyond  the  arms 
at  each  end,  the  length  of  these  heads  being  2^8  ins.  and  their  width  Y*> 
in.  The  heads  of  the  spider  arms  are  turned  off  to  fit  very  closely  in- 
side the  drum,  r,  which  is  mounted  on  the  spider  in  such  a  position  as  to 
bring  the  spider  arms  in  alignment  with  the  lugs,  /,  of  the  drum ;  screws 
through  the  spider  arms  into  the  lugs  hold  the  drum  and  spider  together. 
The  shaft,  S,  is  8}^  ins.  long;  the  portion,  a,  is  2*/2  ins.  long  and  fa 
in.  in  diameter ;  b  is  y2  in.  long  and  J/2  in.  in  diameter ;  the  part  passing 
through  the  core  is  2J/2  ins.  long  and  ^  in.  in  diameter ;  d  is  i  J4  ins.  long 
and  3/2  in.  in  diameter,  and  e  is  i*4  ms<  l°ng  and  Y*  in.  in  diameter.  The 
spider,  s,  should  be  secured  to  the  shaft  by  a  key,  the  key-seat  being  lo- 
cated at  the  base  of  one  of  the  arms.  The  front  end  of  the  commutator 
must  be  located  not  less  than  3-16  in.  from  the  shoulder  where  d  and  c 
join. 

The  armature  is  next  prepared  for  winding  by  removing  the  drum 
and  core  from  the  spider  and  insulating  the  ends  and  interior  of  the  core 
and  the  walls  of  the  slots.  Cut  four  rings  of  heavy  drilling  of  a  size  to 
cover  the  washers,  zv  7Cr,  and  the  ends  of  the  drum,  r ;  varnish  two  of  them 
on  one  side  with  shellac,  and  apply  them  to  the  ends  of  the  armature  body. 
While  these  are  hardening  cut  twenty  strips  of  micanite  cloth,  25-1000  in. 
thick,  \Y*  ms-  wide  and  2  ins.  long;  in  each  end  of  each  of  these  cut  two 
slits,  }4  in.  long,  parallel  with  the  sides  and  located  17-32  in.  from  each 
side  of  the  strip.  Varnish  these  on  one  side,  and  when  nearly  dry  fold 
them  into  troughs  to  fit  the  slots ;  fold  them  so  that  the  varnish  will  be  on 
the  inside  of  the  trough. 

When  these  are  dry  varnish  the  slots  and  the  outsides  of  the  troughs 
and  put  the  latter  in  the  slots,  bending  the  ends  flat  against  the  core  and 
securing  them  there  with  a  little  fresh  varnish.  Then  varnish  the  ends 
of  the  core  (two  cloth  rings  being  on  them),  and  one  side  of  the  two 
remaining  rings  of  drilling;  put  these  rings  on  top  of  the  first  ones, 
varnish  them  on  the  outside  and  put  the  core  in  an  oven  to  bake.  The 
armature  coils  consist  of  No.  22  double  cotton-covered  wire,  wound  seven 
turns  wide  and  thirteen  layers  deep.  Before  winding  them  four  strips  of 
wood  3  ins.  long,  y%  in.  wide  and  3/2  in.  thick  should  be  screwed  to  the  in- 
ner wall  of  the  brass  drum,  in  line  with  the  lugs,  /,  so  as  to  preserve  spaces 
for  the  four  arms  of  the  spider.  A  double  thickness  of  drilling  should  also 


ONE-FOURTH   HORSE-POWER   MOTOR  29 

be  applied  to  the  interior  cf  the  drum  to  insulate  the  coils  from  it.  The 
connections  are  the  simple  Gramme  ring  arrangement.  Before  connecting 
up  to  the  commutator  the  band  wires  should  be  put  on.  Use  No.  19  B. 
W.  G.  soft  tinned-iron  wire,  known  by  hardware  dealers  as  "white  stove- 
pipe wire,"  for  the  bands.,  and  put  them  on  under  as  heavy  pressure  as 
possible  without  endangering  the  armature  shaft.  Two  bands  of  eight 
turns  each,  J/2  in.  from  each  end  of  the  core,  will  suffice.  A  strip  of  mica 
between  two  strips  of  fullerboard  must  go  under  each  band,  and  the  bands 
should  be  soldered  at  intervals,  not  all  the  way  around.  Four  tin  clips 
located  equidistantly,  with  a  dab  of  solder  at  each,  will  give  ample  secur- 
ity. ^ 

The  commutator  (not  shown)  must  be  bored  to  fit  the  l/2  in.  portion, 
d,  of  the  shaft,  and  must  not  exceed  ij4  in.  along  the  shaft;  it  must  have 
a  brush  tread  i  in.  wide.  The  lugs  where  the  wires  are  attached  to  the 
segments  may  project  toward  the  armature  l/§  in.  or  so.  There  must  be 
20  segments,  and  a  diameter  of  2l/2  ins.  is  recommended.  The  quadrant 
carrying  the  brush-holders  should  be  fitted  to  the  inner  end  of  the  journal 
box,  and  carbon  brushes  not  smaller  than  J4  in.  X  H  in.  (one  on  each 
side)  on  the  contact  surface  should  be  used.  If  the  machine' be  used  as  a 
dynamo  (it  will  maintain  five  or  six  no-volt  lamps)  metal  brushes  of  the 
same  surface  should  be  used  to  reduce  the  resistance  of  the  brush  contact. 

The  field  coil  contains  37  layers  of  No.  28  double  cotton-covered 
wire.  After  the  magnet  is  fitted  as  described  in  the  beginning  of  the  arti- 
cle it  is  taken  apart  and  two  circular  magnet  heads  of  fibre  l/%  in.  thick  and 
3^4  ins.  outer  diameter  are  put  on  with  a  driving  fit,  care  being  taken  that 
the  distance  along  the  core  from  outside  to  outside  of  the  heads  cor- 
responds with  the  distance  between  the  pole-pieces  (5  ins.)  when  the 
whole  is  assembled.  A  groove  must  be  cut  on  the  inner  face  of  one  head 
from  the  center  to  the  outer  edge,  in  order  to  lead  out  the  starting  end  of 
the  field  wire,  and  this  must  be  covered  with  two  layers  of  oil  paper  toi 
prevent  short-circuiting  the  successive  layers  of  the  coil.  The  core  must 
be  insulated  with  three  layers  of  shellac  muslin  between  the  headsa  and 
the  field  wire  put  on  evenly,  care  being  taken  not  to  "spread"  the  heads ; 
if  the  winding  is  carefully  done  the  coil  will  be  216  turns  in  length.  The 
number  of  turns  in  length  is  not  a  vital  matter,  but  the  depth  must  be  37 
layers.  The  ampere  turns  are  the  same  no  matter  what  the  length  of  the 
coil,  but  it  should  be  as  long  as  practicable  to  reduce  the  heat  loss. 

After  winding  the  coil  and  securing  the  ends  one  pole-piece  is  put  on 
solid  and  the  other  one  slipped  on  until  it  begins  to  bind,  when  the  journal 
yokes  must  be  inserted  between  their  arms,  and  the  bolts  put  in  as  far  as 
possible  without  jamming.  Then  by  tightening  up  the  journal-yoke  bolts 


30  ELECTRICAL   DESIGNS 

and  the  pole-piece  bolt  together,  being  particular  never  to  draw  the  yoke 
bolt  hard  against  the  arm,  the  frame  will  come  together  in  its  original 
position.  As  an  additional  precaution  it  may  be  set  on  a  true  plar.c  cur- 
face,  and  if  the  base  of  the  loose  pole-piece  gets  out  of  alignment  tup  the 
horn  lightly  until  the  frame  is  true  on  the  bottom.  The  magnet  frame 
must  be  provided  with  a  non-magnetic  base  ;  hardwood  is  as  good  as  any- 
thing, the  frame  being  secured  by  flat-head  brass  machine  screws  from 
below,  two  in  each  casting,  countersunk  in  the  wood. 

The  technical  data  for  the  above  machine  are  as  follows : 


TERMINAL   E.    M.    F.,    IIO   VOLTS. 

Armature  current,  normal 1.9  amps. 

"  "         maximum....     2.3      " 

"          resistance,  warm  ....    4.15  ohms 

Field  current  at  no  volts 25  arr.p. 

"      resistance,  warm 440  ohms 

C^loss  in  field y.-j^/z  watts 

C2  J?  loss  in  armature 15         " 

Hysteresis  loss  in  armature....  48         " 

Magnetic  flux  per  square  inch. 

In  field  core 76,000  lines 

In  pole-pieces , 36,000     " 

In  air-gap 23,000     " 

In  armature  teeth 65,000     " 

In  armature  core 85,000     " 

Co-efficient  of  leakage 1.4 

Electrical  efficiency 82  per  cent. 

Commercial  efficiency  (windage 
and  friction  losses  10  p.  c., 
estimated) 52  " 

Revolutions  per  minute 2,000 


Lamps 


FIG.    26. — DIAGRAM  OF  STARTING  SWITCH. 


It  is  advisable  to  provide  a  starting  switch  similar  to  the  one  shown 
diagrammatically  by  Fig.  26,  where  b  b  are  the  brushes ;  S  the  starting 
switch  lever ;  m  a  magnet,  and  Szv  a  double-pole  snap  switch.  The  lamps 
shown  are  5o-volt,  32-candle-power  lamps.  The  handle  of  the  starting- 
switch  is  provided  with  a  spring  tending  to  keep  it  in  the  position  shown 
by  the  sketch.  This  starting  switch  is  also  suitable  for  use  with  the  motor 
described  in  Chapter  III. 

If  it  is  desired  to  build  a  smooth-core  machine  the  armature  core 
must  be  made  4l/2  ins.  in  diameter,  and  two  grooves  y%  in.  wide  and  5-16 
in.  deep  must  be  cut  in  the  face  of  the  core  at  opposite  points  for  the  re- 
ception of  driving  teeth.  These  are  two  pieces  of  fibre  */6  in.  thick,  l/2 
in.  wide  and  2  ins.  long,  set  on  edge  in  the  grooves  and  projecting  3-16  in. 


ONE-FOURTH  HORSE-POWER  MOTOR  31 

above  the  surface  of  the  core.  The  core  must  be  thoroughly  covered 
with  two  layers  of  micanite  cloth.  The  number  of  coils  is  the  same  as 
before,  but  the  coils  will  be  twenty  turns  wide  and  five  deep  on  the  out- 
side; on  the  inside  of  the  ring  the  coils  must  lap,  making  ten  layers  of 
wire.  The  smooth  core  is  banded  just  as  the  slotted  one  is,  except  that 
soft  brass  wire  must  be  used  instead  of  tinned  iron. 


CHAPTER  V. 


HORSE-POWKR   MOTOR,    WITH   DRUM  ARMATURE. 


For  this  size  of  motor  three  types  of  field  magnet  are  described,  the 
single-coil  Jenny,  like  those  previously  described,  a  bipolar  one-piece 
magnet  of  the  so-called  iron-clad  type,  and  a  similer  form  with  four  poles 


FIG.   27. — END  ELEVATION  OF  FIELD  MAGNET. 

(Kapp  type).  The  armature  core  and  shaft  are  the  same  in  each  case, 
excepting  the  number  of  slots  in  the  four-pole  machine.  The  machine  is 
a  5/2  horse-power  motor  to  operate  on  a  no-volt  constant  potential  circuit 
at  a  speed  of  2,000  revolutions  per  minute.  The  single-coil  magnet 


ONE-HALF  HORSE-POWER   MOTOR 


33 


(Figs.  27  and  28)  has  a  round  core  of  commercial  wrought  iron  2l/2  ins.  in 
diameter  and  n  ins.  long  over  all.  The  ends  are  turned  tapering,  as  in- 
dicated by  the  dotted  lines,  to  insure  intimate  contact  with  the  yokes ;  the 
taper  is  from  the  full  diameter  to  i^  ins.,  and  begins  with  2l/2  ins.  from 
each  end.  The  pole-pieces  are  of  cast-iron.  The  arms  which  support 
the  journal  yokes  are  cast  solid  with  the  pole-pieces,  and  their  horizontal 


L_ 


FIG.    28. — PLAN  AND  FACE  VIEWS  OF  ONE  FIELD-MAGNET  POLE-PIECE. 

thickness  tapers  from  y2  in.  at  the  pole-piece  to  J4  in-  where  the  yoke  is 
bolted  in  place. 

In  fitting  the  magnet  frame  together  the  best  procedure  is  to  bore  the 
tapered  holes  in  the  lower  part  of  each  pole-piece  and  turn  the  ends  at  the 
magnet  core  to  the  same. taper,  but  just  a  trifle  larger;  then  dress  each 


34 


ELECTRICAL  DESIGNS 


tapered  end  of  the  core  down  very  gradually  with  a  fine  file  (the  core  be- 
ing run  on  a  lathe)  until  the  pole-piece  can  be  pushed  on  by  hand  far 
enough  to  bring  the  end  of  the  core  within  3-64  in.  of  the  surface  of  the 
cast-iron.  The  pole-pieces  and  ends  of  the  core  should  be  punch-marked 
so  as  to  insure  finally  mounting  each  pole-piece  on  the  end  which  was 
fitted  to  it.  After  dressing  down  the  ends  of  the  core  as  above  described 
drill  and*  tap  in  each  end  a  hole  for  a  34 -in.  machine  screw,  the  purpose 
of  which  will  be  made  apparent  by  a  glance  at  the  right-hand  end  of 
the  complete  magnet  in  Fig.  27,  where  C  is  a  four-armed  claw  or  spider, 
with  a  hole  through  the  center  where  the  arms  intersect.  The  arms  are 
3-16  in.  thick,  measured  at  right  angles  to  the  bolt,  and  taper  from  3-16 
to  y%  in.  thick,  measured  parallel  with  it.  One  of  these  claws  or  spiders 
is  used  at  each  end  of  the  core,  though  the  drawing  shows  it  at  one  end 
only. 

After  drawing  one  pole-piece  home  solid  by  means  of  the  spider  and 
bolt,  slip  the  other  on  the  other  end 
of  the  core  loosely  and  clamp  the 
pole-pieces  lightly  between  two  iron 
plates  with  planed  surfaces,  applied 
between  the  journal  arms,  so  as  to 
keep  the  four  horns  of  the  pole-pieces 
in  alignment ;  then  force  the  second 
pole-piece  home  and  clamp  the  horns 
hard  between  the  iron  plates.  The 
bottom  surface  of  the  pole-pieces  are 
then  to  be  turned  up  on  a  shaper  or 
planer  and  the  iron  clamping  plates 
removed  from  the  horns. 

The  next  operation  is  boring  the 
armature    chamber    and    the    seats  • 

for  the  ends  of  the  journal  yokes.  The  bore  of  the  armature  chamber  is 
4  3-16  ins.;  the  seats  for  the  journal  yokes  are  machined  to  a  4^-in.  cir- 
cle for  24  m.  from  the  outer  ends.  These  operations  must  be  completed 
before  the  original  position  of  the  frame  on  the  lathe  or  boring  machine 
is  altered.  This  completes  the  machine  work  on  the  magnet,  with  the 
exception  cf  the  holes  through  the  ends  of  the  supporting  arms  and  holes 
in  the  bottom  surfaces  cf  the  pole-pieces  for  bolting  to  the  base. 

The  journal  ycke  must  be  made  cf  brass  or  some  similar  composition. 
The  bar  is  3-16  in.  thick  and  I  in.  wide,  except  near  the  ends,  where  it 
flares  to  correspond  with  the  width  cf  the  supporting  arms.  At  each  end 
is  a  right-angle  lug,  */g  in.  thick  after  machining ;  these  lugs  fit  the  ma- 


FIG.    29. — JOURNAL  YOKE. 


ONE-HALF  HORSE-POWER  MOTOR  35 

chined  seats  in  the  ends  of  the  iron  arms,  and  the  yokes  should  be  fitted 
to  these  arms  before  the  frame  is  taken  apart  to  put  on  the  magnet  coil. 
The  journal  box  is  !J/2  ins.  long  over  all,  3-16  in.  of  its  length  projecting 
on  the  inside  of  the  yoke  bar,  and  ij/g  ins.  on  the  outside.  As  shown  by 
the  plan  view  of  the  yoke  in  Fig.  29,  there  are  stiffening  webs  starting 
flush  near  the  ends  of  the  yoke  and  attaining  a  height  of  ft  in.,  where  they 
join  the  box ;  these  ribs  are  %  in.  thick.  The  box  is  I  in.  in  outside  diam- 
eter and  bored  out  9-16  in.;  the  bore  is  bushed  to  ^  m-  No  particular 
form  of  oiling  device  is  specified,  as  any  amateur  of  sufficient  ability  to 
build  such  a  motor  will  be  fully  competent  to  decide  this  detail  for  him- 
self. The  journal  yokes  are  held  in  place  by  %-in.  cap-screws  passing 
through  the  ends  of  the  supporting  arms  and  tapping  into  the  lugs  on  the 
yokes. 

The  field  coil  contains  35  layers  of  No.  26  double  cotton-covered 
magnet  wire.  After  the  magnet  is  fitted  as  above  described  it  is  taken 
apart  and  two  circular  fibre  heads  4^2  ins.  in  diameter  and  J^  in.  thick 
are  put  on  the  core  with  a  driving  fit,  care  being  taken  that  the  distance 
from  outside  to  outside  of  the  heads  corresponds  with  the  space  between 
the  perpendicular  faces  of  the  pole-pieces  when  the  frame  is  assembled ; 
this  measurement  should  be  taken  prior  to  dismantling  the  frame.  A 
groove  must  be  cut  on  the  outer  face  of  one  head,  from  the  center  to  the 
outer  edge,  in  order  to  form  a  channel  for  leading  out  the  starting  ends 
of  the  coil  when  the  frame  is  re-assembled,  at  which  time  two  discs  of  oil 
paper  with  one  of  mica  between  them  must  be  threaded  on  the  core  out- 
side cf  this  head  to  insulate  the  leading-out  wire  from  the  pole-piece.  Be- 
fore winding  the  coil  insulate  the  core  with  a  strip  of  muslin  just  wide 
enough  to  go  between  the  heads,  and  long  enough  to  wrap  around  the 
core  three  times ;  this  should  be  heavily  shellacked  before  it  goes  on. 
If  the  coil  is  carefully  wound  it  will  be  210  turns  in  length  along  the  core; 
the  umber  of  turns  in  this  direction  is  not  particularly  essential,  but  as 
many  should  be  put  on  as  possible  without  jamming  the  insulation,  in 
order  to  reduce  the  heat  loss.  The  depth  of  the  winding  must  be  35 
layers. 

After  winding  the  coil  and  securing  the  ends,  put  one  pole-piece  on 
solid  and  slip  the  other  on  loosely.  When  it  begins  to  bind  bolt  the  jour- 
nal yoke  to  the  lugs  on  the  pole-piece  first  put  on,  and  insert  the  bolts 
through  the  lugs  of  the  one  that  is  loose.  Then  tighten  up  the  spider  bolt 
at  the  end  of  the  core  and  force  it  into  place,  the  bolts  through  the  lugs 
serving  as  guides  to  keep  the  pole-piece  from  twisting  on  the  core. 
These  bolts  should  be  set  up  little  by  little  with  the  spider  bolt,  so  as  to 
keep  the  bolt  heads  within  1-16  in.  of  the  surface  of  the  lugs.  As  an  ad- 


36  ELECTRICAL  DESIGNS 

ditional  precaution  the  frame  may  be  set  on  a  true  surface  and  tried  at  in- 
tervals to  see  if  it  gets  out  of  alignment ;  if  it  does,  tap  the  horn  of  the 
loose  pole-piece  until  the  bottom  surface  agrees  with  the  guide.  The 
magnet  frame  must  be  provided  with  a  non-magnetic  base,  preferably 
composition  metal,  but  allowably  of  wood. 

Figs.  30,  31  and  32  show  an  armature  disc,  the  shaft  and  armature 
core  (the  latter  in  cross-section),  and  the  shell  and  head.  The  discs  are 
of  charcoal  iron,  4  ins.  outside  diameter,  with  a  I  in.  hole  in  the  center  and 
an  y%  in.  key-seat,  annealed  after  punching  and  key-seating;  there  are 
1 8  slots  3/£  in.  wide  and  3/g  In.  deep.  The  shell  and  one  head  are  cast  in 
one  piece  (of  brass),  and  consist  of  a  barrel  i  in.  outside  diameter  (when 
finished),  and  2  ins.  long,  with  a  head,  j,  at  one  end,  3^  ins.  in  diameter 
and  tapered  in  thickness  from  J4  in.  near  the  center  to  1-16  in.  at  the 
periphery ;  at  the  opposite  end  of  the  barrel  is  a  cross-bar  y&  in.  thick,  cast 


FIG.    31. — ARMATURE  SHAFT  AND  AXIAL  SECTION  OF  ARMATURE  CORE. 

with  the  barrel  and  of  the  shape  shown,  being  j£  in.  wide  where  it  joins 
the  barrel  and  $4  in.  at  the  center.  A  ^2 -in.  hole  is  drilled  in  the  center  of 
this  cross-bar  and  another  in  the  center  of  the  head,  s,  at  the  other  end  of 
the  barrel ;  the  shell  is  mounted  on  a  mandrel,  the  barrel  is  turned  down 
to  fit  the  hole  in  the  armature  discs,  and  both  sides  of  the  head  are  faced 
off  smooth.  A  J^  in.  key-seat  3-16  in.  deep  is  cut  in  the  barrel,  so  as  to 
come  in  the  center  of  one  end  of  the  cross-bar,  as  shown ;  a  l/&  in.  X  1A 
in.  feather,  or  parallel  key  is  laid  in  the  key-seat,  and  the  discs  are  thread- 
ed on  the  barrel  and  compressed  against  the  head  by  the  collar,  h,  drawn 
down  by  two  bolts  (not  shown)  passing  through  the  collar  and  in- 
side the  barrel,  and  tapping  into  the  head  at  the  other  end.  This  collar, 
^,  is  of  brass,  3^3  ins.  in  diameter  and  tapering  from  3-16  to  1-16  in.  in 
thickness  when  finished.  The  opening  in  the  center  should  fit  the  outline 
of  the  cross-bar  on  the  end  of  the  barrel  at  least  closely  enough  to  pre- 


ONE-HALF  HORSE-POWER  MOTOR 


37 


vent  the  collar  from  shifting  under  stress  of  centrifugal  force ;  the  collar 
must  be  finished  up  smooth  on  both  sides.  A  disc  of  insulation  should 
be  put  on  next  to  the  brass  head  before  the  iron  discs  are  put  on,  and 
another  insulating  disc  should  go  between  the  last  iron  disc  and  the 
clamping  collar,  h. 

If  the  slots  are  cut  in  the  core  with  a  milling  machine  the  discs  must 
all  come  off  the  barrel  to  have  the  burrs  removed,  and  also  be  re- 
annealed  ;  the  key-seat  will  insure  their  returning  in  the  original  angular 
position,  It  is  much  better  to  have  discs  with  the  slots  punched  before 
the  first  annealing.  The  shaft  is  10^4  ins.  long  over  all ;  y2  in.  in  diam- 
eter in  the  largest  part ;  7-16  in.  where  the  commutator  goes,  and  Y%  in. 
in  the  journals.  A  1-16  in.  X  Yz  in.  collar,  e,  is  shown  back  of  the  arma- 
ture, the  purpose  of  which  is  merely  to  "locate"  the  armature  shell ;  it  is 
not  absolutely  necessary,  however,  and  may  be  left  off  if  desired.  The 


FIG.   30. — ARMATURE  DISC. 


FIG.    32. — ARMATURE   CORE   DRUM  AND   HEAD. 


easiest  way  to  provide  for  it  is  to  make  the  shaft  of  ^  in.  stock,  leaving 
the  original  metal  to  form  the  collar  when  turning  the  shell  to  the  proper 
diameter.  The  armature  shell  may  be  keyed  to  the  shaft  or  pinned 
obliquely  through  the  thick  part  of  the  head ;  it  must  be  positively  secured 
by  some  such  means. 

The  commutator  shell  must  be  bored  to  fit  the  7-16  in.  portion  of  the 
shaft,  and  must  not  exceed  ij4  ins.  along  the  shaft.  The  lugs  where  the 
wires  are  attached  to  the  segments  may  project  toward  the  armature  3/£ 
in.  or  so.  There  must  be  18  segments,  and  a  diameter  of  2  ins.  is  rec- 
ommended. The  quadrant  carrying  the  brushholders  should  be  fitted 
to  the  inner  end  of  the  journal  box,  and  carbon  brushes  (one  on  each  side) 
not  smaller  than  J4  in-  X  /^  in.  on  the  contact  surface  should  be  used. 
If  the  machine  be  used  as  a  dynamo  (it  will  maintain  about  ten  no-volt 
lamps)  metal  brushes  of  the  same  surface  should  be  used  to  reduce  the 


.38  ELECTRICAL  DESIGNS 

resistance  of  the  brush  contact.  The  armature  winding  is  divided  into 
18  coils,  each  having  32  turns  of  No.  20  double  cotton-covered  wire,  eight 
turns  wide  and  four  turns  deep  in  the  slot.  The  slots  must  be  insulated 
with  troughs  of  muslin  and  mica,  or  preferably  flexible  micanite,  0.03  in. 
thick.  The  troughs  are  easily  made  by  cutting  the  material  into  strips 
2^/2  ins.  long  by  ij.^s  ins.  wide,  and  slitting  the  ends  so  as  to  permit  the 
projecting  portion  of  the  trough  to  be  folded  back  flat  against  the  core. 
Before  putting  in  the  troughs  a  disc  of  heavy  drilling  3^4  ins.  in  diam- 
eter should  be  secured  to  each  end  of  the  core  by  means  of  varnish,  and 
the  outer  faces  varnished  and  allowed  to  nearly  dry.  Then  put  in  the 
troughs  and  put  on  two  more  muslin  discs,  varnishing  the  whole,  and 
bake  until  thoroughly  dry.  Instead  of  winding  each  coil  in  diametrically 
opposite  slots,  take  slots  lacking  one  of  being  precisely  opposite. 

A  good  plan  is  to  make  a  sketch  of  an  armature  disc  and  number  the 
slots  from  left  to  right  successively  around  the  periphery.  Then  wind 
the  coils  as  follows,  the  coil  numbers  indicating  the  order  in  which  the 
coils  are  put  on,  not  the  order  in  which  they  are  connected  to  the  commu- 
tator : 

COIL  NO. — i      2      3     4     5     6     7     8     9   10   n    12    13    14     15     16     17     18 

STARTS  IN  SLOT  NO.— I   IO   13   4   7  l6   2  II  1$   6  14   5  iS   Q    3   12    8   17 

ENDS  IN  SLOT  NO. — 9    18      3    12    15     6   io     i     5    14     i    13     8    ij     ii       2     16       7 

Each  pair  of  coils  must  be  covered  with  muslin  where  they  cross  the 
lieads  before  the  next  pair  is  put  on,  and  before  coil  No.  8  is  wound  on  top 
of  coil  No.  i  in  slot  No.  i  the  bottom  coil  must  be  insulated  by  a  strip  of 
inicanite  laid  in  the  slot ;  this  is  true  of  every  bottom  coil. 

After  the  winding  is  on,  and  before  connecting  up  to  the  commutator, 
the  band  wires  should  be  put  on.  Use  No.  19  B.  W.  G.  soft  tinned-iron 
wire,  known  by  hardware  dealers  as  "white  stove-pipe  wire,"  for  the 
Lands,  and  put  them  on  under  as  heavy  pressure  as  possible  without  en- 
dangering the  armature  shaft.  Two  bands  of  eight  turns  each,  y2  in.  from 
•each  end  of  core  will  suffice.  A  strip  of  mica  between  two  strips  of  fuller- 
Loarcl  must  go  under  each  band,  and  the  bands  should  be  soldered  at  in- 
tervals, not  all  the  way  around.  Four  tin  clips  located  equidistantly,  with 
a  dab  cf  solder  at  each,  will  give  ample  security. 

If  cast  steel  be  available,  one  of  the  iron-clad  types  of  magnet,  shown 
l>y  Figs.  33  and  34,  is  somewhat  preferable  because  of  the  small  amount  of 
machine  work  required.  Of  these  two  the  four-polar  type  is  considered 
preferable  by  the  writer,  being  much  lighter  in  weight  and  having  an 
"open-head"  armature  winding.  Each  of  the  iron-clad  magnets  is  a 
single  casting;  the  essential  dimensions  are  shown  in  the  sketches,  with 


ONE-HALF  HORSE-POWER  MOTOR 


39 


t" 


•n: — mfi 


| 


u 


a 

B 
S 


o 

it- 


i 


T 


i  - 


i 


40  ELECTRICAL  DESIGNS 

the  exception  of  the  bore  of  the  armature  chamber,  which  is,  of  course, 
the  same  as  for  the  single-coil  magnet — 4  3-16  ins.  As  the  two  mag- 
nets require  the  same  treatment,  varying  only  in  dimensions,  the  fol- 
lowing remarks  apply  to  both  : 

It  will  be  noticed  that  the  feet  of  the  -machine  project  y%  in.  below 
the  body  and  that  there  is  a  transverse  rib  under  the  center  of  similar 
depth.  These  are  to  give  the  machine  a  floor  bearing  which  may  be 
trued  up  on  a  shaper  or  planer  without  finishing  the  whole  bottom  of  the 
machine.  The  first  operation  on  the  casting  is  chipping  off  the  numerous 
fins  and  lumps  with  which  steel  castings  are  invariably  afflicted.  An  em- 
ery wheel  may  be  used  for  this  purpose  around  the  outside  of  the  frame, 


t 


• 


FIG.  34. — SIDE  ELEVATION  OF  FOUR-POLE   IRON-CLAD  FIELD  MAGNET. 

but  in  the  corners  of  the  coil  spaces  a  cape  chisel  and  lots  of  muscular 
exertion  will  be  required. 

Next,  the  bearing  surfaces  are  trued  up,  and  J^-in.  holes  drilled 
in  the  feet;  then  the  magnet  is  mounted  for  boring  out  the  armature 
chamber  and  the  seats  for  the  journal  yokes,  all  of  which  must  be  done 
with  one  mounting.  This  finishes  the  magnet  frame,  unless  it  is  de- 
sired to  put  a  terminal  block  on  the  machine  instead  of  on  the  base 
and  do  away  with  the  latter.  In  this  event  four  ^-in.  holes  are  to  be 
drilled  in  the  top  surface  of  the  frame  and  tapped  for  machine  screws  to 
hold  the  block,  which  may  be  2  by  6  ins.  and  ijX  ins.  thick.  The  journal 
yoke  and  journal  are  the  same  as  shown  in  Fig.  29,  except  that  for  the  bi- 
polar magnet  the  yoke  is  7^  ins.  long  instead  of  4^/8  ins. 

The  field  coils  for  the  bipolar  machine  consist  of  No.  25  double  cot- 
ton-covered wire  wound  45  layers  deep,  'and  each  coil  is  80  turns  lonj. 


ONE-HALF  HORSE-POWER   MOTOR 


The  coils  are  to  be  wound  in  fibre  bobbins,  as  shown  by  Fig.  35.  The 
heads  of  the  bobbin  must  be  2%  ins.  apart,  and  the  body  must  be 
Y§  in.  wider  and  longer  than  the  magnet  core,  actual  measurement.  Be- 
fore winding  the  coil  the  bobbin  must  be  mounted  on  a  wooden  core  of 
proper  size  to  fit  the  opening  through  the  center,  and  having  flanges  or 
heads  at  each  end  to  "back  up"  the  heads  of  the  bobbins;  one  of  these 
heads  is  put  on  permanently  and  the  other  is  secured  by  two  screws 
so  as  to  be  removable.  A  spindle  of  i-in.  iron  goes  through  the  center  of 
the  wooden  core  upon  which  to  mount  it  in  the  lathe  for  winding. 

When  a  coil  is  completed  bend  the  wire  back  upon  itself  near  the 
end,  tie  a  linen  thread  in  the  loop  formed,  and  secure  the  end  of  the 
coil  by  passing  the  thread  several  times  around  the  coil  and  tying  its 
ends  together.  Then  varnish  the  outside  heavily  and  bake  the  coil  at  a 
low  temperature — 100  to  125  deg.  Fah. — until  the  varnish  is  hard. 

The  coils  for  the  four-polar  machine  are  35  layers  deep,  and  no  long, 


FIG.    35. — MAGNET  COIL    BOBBIN. 


FIG.   36. — ARMATURE  DIAGRAM. 


of  No.  25  wire.  The  heads  of  the  winding  bobbin  are  3  ins.  apart.  The 
instructions  for  winding  the  coils  for  the  bipolar  iron-clad  machine 
apply  to  these  also.  In  connecting  the  coils  on  the  machine,  however, 
there  is  a  difference.  On  the  bipolar  machine  the  final  end  of  one  coil 
must  be  connected  to  the  beginning  of  the  other ;  on  the  quadripolar  the 
reverse  is  true.  Fig.  37  shows  diagrammatically  the  manner  of  connect- 
ing the  field  coils  of  the  quadripolar  machine.  It  will  be  noticed  that 
the  exciting  current  passes  around  the  cores  in  opposite  directions.  The 
connection  for  the  bipolar  machine  is  exactly  the  reverse  of  that  shown. 
The  armature  core  of  the  four-pole  machine  has  19  slots  3-10  in.  wide 
and  y*  in.  deep,  instead  of  18  slots  3/sXH-  There  are  19  coils,  each  hav- 
ing 28  turns  of  No.  20  wire,  7  turns  wide  and  4  turns  deep.  These  may  be 
wound  directly  on  the  core,  but  it  will  probably  be  easier  for  an  amateur 
to  wind  them  in  a  little  frame,  tie  them  at  intervals  with  thread  and  put 


42  ELECTRICAL   DESIGNS 

them  on  the  core  complete.    The  winding  frame  will  be  exactly  like  the 
one  for  the  field  coils  except  in  size.     The  "channel"  formed  between 


FIG.    37. — DIAGRAM   OF  FOUR-POLE   FIELD-COIL  CONNECTIONS. 

the  heads  must  be  9-32  in.  wide  and  J4  m  -deep.     The  body  of  the  frame, 
determines  the  length  and  width  of  the  coil,  is  2j4  ms-  one  way 


TABLE   OF  WINDING  AND   CONNECTIONS. 


NUMBER  OF 
COIL 

IN    SLOTS 
NOS. 

BEGINNING  END 
GOES  TO  SEG- 
MENT  NUMBER 

FINAL   END 
GOES  TO  SEG- 
MENT NO. 

I 

I    and     6 

I 

II 

2 

2               7 

2 

12 

3 

3               8 

3 

13 

4 

4              9 

4 

14 

5 

5             10 

5 

15 

6 

ii              16 

ii 

2 

7 

12             17 

12 

3 

8 

13         is 

13 

4 

9 

13             19 

14 

5 

10 

15                I 

15 

6 

ii 

16               2 

16 

7 

12 

17               3 

17 

8 

13 

18              4 

18 

9 

14 

19               5 

19 

10 

15 

6             ii 

6 

16 

16 

7            12 

7 

17 

17 

8             13 

8 

18 

18 

9            14 

9 

19 

19 

10            15 

10 

I 

and  2}4  ins.  the  other.     The  coils  are  put  on  and  connected  up  as  indi- 
cated by  the  accompanying  table. 

The  numbers  of  the  coils  indicate  the  sequence  in  which  they  are  put 


ONE-HALF  HORSE-POWER  MOTOR  45 

on  the  core,  and  this  order  should  be  observed  in  order  to  secure  maxi- 
mum symmetry  of  the  wires  across  the  heads  of  the  core.  The  numbers- 
of  the  slots  and  segments  refer  to  the  diagram  shown  by  Fig.  36.  Each 
figure  applies  to  the  slot  and  segment  between  which  it  is  located. 

The  brush  quadrant  for  this  machine  is  also  different  from  that  of 
the  other  two ;  instead  of  bearing  upon  the  commutator  at  diametrically 
opposite  points,  the  brushes  must  be  90  deg.  apart — corresponding; 
with  the  relative  angular  positions  cf  magnet  poles  of  different  signs.  In 
the  bipolar  iron-clad  the  "north"  and  "south"  poles  are,  of  course,  oppo- 
site each  other ;  in  the  four-pole  machine  the  poles  directly  opposite  are 
of  the  same  sign — if  one  horizontal  pole  is  "north"  the  other  must  also  be 
"north,"  and  the  other  two,  without  coils,  will  be  "south." 


CHAPTER  VI. 


ONE  HORSE-POWER   BIPOLAR   MOTOR,    WITH   DRUM  ARMATURE. 


The  accompanying  drawings  and  description  will  enable  any  one 
\vith  moderate  machine-shop  facilities  to  build  a  1  horse-power  motor 
to  work  on  a  no-volt  or  a  22O-volt  continuous-current  circuit.  Two 
types  of  field  magnet  are  given,  the  armature  and  shaft  being  the  same 
in  both  cases. 

The  armature  is  4  ins.  in  diameter,  outside,  with  twenty-four  slots, 
each  7-32  in.  wide  and  y%  in.  deep.  Fig.  38  shows  the  shaft  and  a  cross- 
sectional  view  of  the  armature  core.  The  discs  are  compressed  by  two 
cast-iron  end  plates,  which  are  screwed  on  the  shaft ;  these  plates  are  J/2 
in.  thick  at  the  shaft,  and  taper  to  3-16  in.  thick  at  the  outer  edge,  which 


FIG.    38. — ARMATURE  SHAFT  AND  AXIAL   SECTION   OF  ARMATURE   CORE. 

is  rounded  as  shown,  to  avoid  abrading  the  insulation  between  the  core 
and  the  windings.    The  full  list  of  armature  dimensions  is  as  follows : 

Core  Shaft 

Body.     Heads.  at  1.  m.  n.  p.              q. 

Diameter 4             2^  #  #  i  ^             y2 

Axial  length 4              j£  5  i#  5  3^          2 

The  discs  should  have  a  shallow  key-seat  in  the  edge  of  the  central 
hole,  and  the  shaft  should  be  correspondingly  key-seated,  and  a  spline, 
or  perfectly  straight  key,  J^-in.  square,  should  be  used  to  transmit  the 
movement  of  the  discs  to  the  shaft.  If  this  is  done,  the  slots  in  the  per- 


ONE  HORSE-POWER  BIPOLAR   MOTOR 


45 


iphery  of  the  discs  may  be  milled  out;  the  armature  core  must  be  dis- 
mantled after  the  slots  are  cut,  and  the  burr  which  is  left  by  the  milling 
center  smoothed  off.  If  the  key  and  key-seats  are  properly  fitted  the 
discs  will  go  back  on  the  shaft  in  precisely  the  position  which  the  slots 
were  cut,  and  the  sides  of  the  latter  will  be  smooth.  If  the  key  is  a  loose 
fit,  however,  it  will  be  advisable  to  use  a  straight  edge  in  one  of  the  slots 
to  insure  perfect  accuracy  in  re-assembling  the  discs.  It  is  scarcely 
necessary  to  urge  a  very  careful  and  close  fit  of  the  key  and  its  seats.  In 
assembling  the  core,  one  of  the  cast-iron  heads  should,  of  course,  be 
screwed  to  place  first;  then^put  on  a  disc  of  vulcanized  fibre,  1-16  in. 
thick,  4  ins.  in  diameter,  and  next  thread  on  the  iron  discs.  After  the 
last  iron  disc  put  on  another  fibre  disc  and  follow  with  the  end  plate  or 


< J *< --K-- -*-•» 

FIGS.    39  AND  40. — PLAN  VIEW  AND  END  ELEVATION  OF  IRON-CLAD  MAGNET. 

head  of  cast-iron,  which  will  have  to  be  set  up  with  a  pin  wrench.  If  the 
discs  are  purchased  with  the  slots  already  stamped  out  notches  will  have 
to  be  cut  in  the  fibre  end  discs  to  correspond  with  the  armature  slots ;  if 
the  slots  are  to  be  milled  the  fibre  discs  will,  of  course,  be  cut  along  with 
the  iron  ones.  The  latter  must  be  not  over  1-32  in.  thick  and  preferably 
thinner;  care  should  be  taken  not  to  get  steel  discs,  but  the  very  best 
possible  grade  of  charcoal  iron. 

Of  the  two  types  of  field  magnets  shown,  the  iron-clad  is  preferable 
from  a  constructional  standpoint,  as  the  only  operations  are  boring  out 
the  armature  chamber  and  the  seats  for  the  journal  pedestals,  and  drill- 
ing the  bolt  holes  for  the  latter.  Fig.  39  gives  a  plan  view  of  the  iron- 


46 


ELECTRICAL   DESIGNS 


clad  magnet,  Fig.  40  an  end  view  and  Fig.  41  a  side  elevation.  The 
thickness  of  the  magnet  core  (the  portion  on  which  the  coils  are  placed) 
parallel  with  the  shaft  is  4%  ins.  except  right  at  the  pole  face,  where  it 
is  rounded  down  to  4  ins. ;  this  is  necessary  in  order  to  reduce  the  flow 
of  magnetism  from  the  pole  to  the  cast-iron  end  plates  of  the  armature, 
which  produces  waste  of  energy  by  heating.  The  complete  measure- 
ments of  the  field  magnet  are  as  follows : 

INCHES. 

A — Thickness  of  yoke  portion  of  magnet i% 

B — Inside  length  of  horizontal  part  of  yoke 8 

C — Vertical  thickness  of  magnet  core 4^ 

D — Distance  from  core  to  yoke 2^ 

E — Total  outside  width  of  magnet  frame II 

F — Width  of  journal  foot 3 

G — Radius  to  which  journal  scat  is  bored 4^ 

H — Horizontal  thickness  of  magnet  core  (see  above) 4^" 

J — Length  of  journal  foot,  commutator  side 4^ 

K — Width  of  magnet  yoke  or  frame,  axially 8j£ 

L — Length  of  journal  foot,  pulley  side 2^i 

The  bore  of  the  pole  pieces  is  4  3-16  in.  in  diameter,  and  this  figure 
must  be  rigidly  observed  for  best  results,  as  all  the  calculations  are  based 
upon  this  length  of  air-gaps.  The  above  dimensions  are  intended  to 

apply  to  a  magnet  made  of  the  best      ^ -N. 

grade  of  cast-iron;  Scotch  pig  should 
be; used  if  it  is  obtainable,  and  if  not, 
then  the  very  best  grade  of  soft/iron. 
The  casting  should  be  allowed  to  re- 
main in  the  mold  until  it  is  absolute- 
ly cold  care  being  taken  not  to  remove 
any  of  the  sand  from  about  the  mag- 
net proper.  The  sand  can  be  scraped 
away  from  the  extreme  end  of  the 
longer  of  the  two  pedestal  feet,  so  as 
to  enable  the  molder  to  ascertain 
when  the  casting  is  cold.  It  is  fre- 
quently the  case  that  a  casting  re- 
quires as  much  as  two  days  to  thor- 
oughly cool,  but  it  should  not  be 

...  -,   ,       r  ...  11  FIG.    41. — SIDE   VIEW   IRON-CLAD   MAGNET. 

disturbed  before  it  is  cold. 

Fig.  42  gives  outside  and  cross-sectional  views  of  the  journal  pedes- 
tal for  this  magnet;  the  two  pedestals  are  alike  in  every  particular,  and 
when  in  position  on  the  projecting  feet  of  the  field  magnet  frame  their 


ONE  HORSE-POWER  BIPOLAR  MOTOR 


47 


outer  edges  should  be  exactly  flush  with  the  ends  of  the  feet.  The 
pedestals  are  of  iron;  the  base  is  curved  to  conform  to  the  arc  of  the 
circle  to  which  the  upper  surface  of  the  foot  is  machined,  and  is  j£  inch 
thick.  The  standard  consists  of  two  ribs  at  right  angles  with  each  other, 
each  3^  in.  thick,  with  their  edges  curved  as  shown.  The  box  is  of  the 
ring-oiling  type,  with  a  single  ring  hung  midway  of  the  journal;  the 
bushing  is  easily  made  from  thin  brass  tubing,  J^  in.  outside  diameter, 
and  with  a  very  thin  wall  (not  over  1-32  in.),  babbitted  to  fit  the  shaft 
and  having  a  slot  y%  in.  wide  cut  half  way  through  it,  midway  between  its 
ends.  This  bushing  is  shown  in  Fig.  43,  which  represents  the  bearing 
for  the  other  type  of  magnet,  to  be  presently  described.  The  bushing 
is  134  ins.  long;  the  oil  ring  is  made  of  brass,  one  inch  in  diameter,  in- 
side, \}i  ins.  diameter  outside,  and  }4  in.  wide  along  the  shaft.  Refer- 
ence to  the  side  views  of  the  journal  pedestal  will  show  a  slot  in  the 


FIG.    42. — DETAILS    OF   JOURNAL   BOX   AND    PEDESTAL. 


FIG.   43. — JOURNAL  YOKE 
FOR  FIG.  44. 


upper  wall  of  the  box  portion,  through  which  the  oil  ring  is  inserted 
before  putting  in  the  bushing.  A  cover  should  be  provided  for  this  slot 
to  keep  out  dust,  etc.  The  dimensions  of  the  journal  pedestals  are  as 
follows : 

INCHES. 

g — Length  of  base  and  journal  box 2 }£ 

h — Width  of  base 3 

j — Outer  diameter  of  reservoir 2 

k — Axial  length  of  reservoir,  outside I  % 

Internal  diameter  of  reservoir 2^f 

Internal  length  of  reservoir I 

The  bore  of  the  box  portion  of  the  pedestal  must,  of  course,  be 
made  to  fit  snugly  the  outer  diameter  of  the  tubing  used  for  a  bushing, 
as  the  wall  of  the  latter  is  too  thin  to  admit  of  turning  it  down  to  fit  a 
predetermined  bore  in  the  pedestal.  After  boring  the  pedestal  to  fit  the 
bushing  it  should  be  mounted  on  a  mandrel  and  its  base  turned  to  fit  the 


48 


ELECTRICAL  DESIGNS 


circle  of  the  foot  on  the  magnet  frame,  namely,  9^  ins.  in  diameter. 

Each  pedestal  should  be  fastened  to  the  foot  with  two  y\ -in.  cap  screws. 
Fig.  44  gives  a  side  elevation  of  a  much  lighter  magnet,  which  may 

be  used  in  connection  with  the  armature  above  described,  if  the  builder 

has  sufficient  skill  and  facilities  to  do  the  machine  work  accurately. 

The  magnet  core  is  a  round  piece  of 
wrought  iron,  3^  ins.  in  diameter, 
with  its  ends  turned  down  to  3^  ins. 
diameter  for  a  distance  of  4  ins.  from 
each  end;  the  total  length  of  the 
core  is  12^  ins.,  so  that  the  length 
of  the  untouched  portion  will  be  4^ 
ins.  The  pole  pieces  are  of  cast- 
iron,  only  the  very  best  possible 
grade  being  suitable.  Where  the  core 
enters  the  cast-iron  the  latter  is  4  ins. 
square,  with  the  corners  rounded, 
and  having  two  ribs  or  flanges,  /,  /, 
running  along  one  edge;  these  con- 
tinue clear  up  to  the  top  of  the  pole 
piece,  and  are  i  in.  thick  by  2  ins. 


B •* 

FIG.   44. — SINGLE-COIL  MAGNET 


FIGS.  45  AND  46. — END  ELEVATION  AND  PLAN  VIEW  OF  SINGLE-COIL  MAGNET. 

wide.  Fig.  45  shows  an  end  view  of  the  magnet  frame,  and  Fig.  46  a 
plan  view.  The  hole  occupied  by  the  wrought  iron  core  should  be  cored 
out  to  2%  ins.  diameter  when  the  casting  is  made,  and  afterward  bored 
to  a  driving  fit  of  the  end  of  the  core. 


ONE  HORSE-POWER   BIPOLAR  MOTOR  49 

The  first  operation  should  be  turning  off  the  ends  of  the  core ;  next, 
bore  the  holes  in  the  pole  pieces  (or,  more  strictly  speaking,  the  yokes). 
Then  drill  a  ^-in.  hole  through  the  yoke  just  below  the  lower  edge  of 
the  big.  hole  and  at  right  angles  with  it,  to  accommodate  the  clamping 
bolt  shown  in  Fig.  45.  Next  drive  one  end  of  the  core  into  one  yoke 
and  set  up  the  nut  on  the  end  of  the  clamping  bolt ;  then  put  on  the  other 
yoke  and  twist  it  on  the  core  until  the  four  horns  of  the  pole  pieces  are 
exactly  opposite  each  other,  tighten  up  the  second  clamping  bolt,  and 
plane  off  the  bottom  surfaces  of  both  yokes.  To  bring  the  pole  horns 
into  alignment,  the  simplest  method  is  to  cut  out  two  heavy  blocks  of 
hard  wood,  say  3  ins.  thick  and  3^  ins.  square;  bore  a  J^-in.  hole 
through  the  center  of  each  block,  run  a  y2-m.  bolt,  12  ins.  long,  through 
the  two  blocks,  and  apply  them  to  each  side  of  the  pole  pieces,  the  bolt 
passing  through  the  armature  chamber  in  about  the  position  to  be  occu- 
pied by  the  shaft.  Set  up  the  nut  on  the  bolt  until  the  blocks  are  hard 
against  all  four  pole  horns,  and  then  tighten  up  the  clamping  bolt  in  the 
foot  of  the  loose  yoke. 

After  planing  off  the  feet  of  the  frame,  bore  out  the  armature  cham- 
ber 4  3-16  ins,  in  diameter,  and  the  seats  for  the  journal  yokes  (in  oppo- 
site faces  of  the  side  flanges,  /,  /),  5  ins.  in  diameter,  and  then  remove 
one  magnet  yoke  and  put  on  the  magnet  coil.  If  the  coil  is  separately 
wound  in  a  form  (which  is  preferable)  only  one  yoke  need  come  off ;  if  it 
is  wound  directly  upon  the  core,  both  yokes  must  come  off,  of  course,. 
The  base  of  the  machine  must  be  of  wood  or  brass.  Wood  is  better,  as, 
aside  from  its  cheapness,  it  affords  convenient  space  for  the  terminal 
posts  and  fuse-block  of  the  machine.  The  base  should  be  15  ins.  X  18 
ins.,  made  of  two  pieces  of  hard  wood  each  ij£  ins.  thick,  glued  and 
screwed  together  with  the  grains  at  right  angles.  The  longer  dimen- 
sion of  the  base  is  to  go  parallel  with  the  shaft,  and  the  machine  should 
be  so  set  as  to  allow  the  pulley  to  overhang  the  edge  of  the  base-board. 

The  pulley  should  be  4  ins.  in  diameter  and  2  ins.  wide  on  the  face ; 
the  latter  should  be  crowned.  The  pulley  should  preferably  be  keyed 
to  the  shaft,  with  a  set-screw  in  the  pulley  hub  on  top  of  the  key.  If 
only  a  set-screw  be  used  to  hold  the  pulley  on  the  shaft,  a  "flat"  must 
be  filed  on  one  side  of  the  shaft  under  the  point  of  the  set-screw. 

The  journal  box  and  yoke  for  this  magnet  is  shown  by  Fig.  43.  It 
must  be  made  of  brass  or  some  other  non-magnetic  composition.  The 
design  and  dimensions  of  the  oil  reservoir,  journal  box  and  bushing  are 
exactly  the  same  as  those  given  for  the  journal  box  of  the  iron-clad  mag- 
net above.  All  the  dimensions  are  given  in  the  following  list,  along 
with  those  of  the  magnet  just  described. 


50  ELECTRICAL  DESIGNS 

INCHES. 

A — Distance  between  yokes 4l/{ 

B — Thickness  of  yoke 4 

C — Radius  of  outer  curve  of  pole-piece 4^ 

D — Length  of  pole  horn I  Y% 

E — Distance  from  pole  horn  to  center  of  magnet  core 3^ 

F — Distance  from  floor  line  to  center  of  magnet  core 3j6 

G— Width  of  foot 2% 

H — Width  of  slot  under  core  hole  in  yoke i  Yz 

] — Width  of  yoke 4 

K— Width  of  flange 2 

a — Diameter  of  curve  of  journal  yoke  ends  and  seats 5 

b — Vertical  width  of  journal  yoke  arms 2  }£ 

c — Length  of  machined  portion  of  yoke  arms i^ 

d — Distance  from  end  of  yoke  arm  to  inner  end  of  journal  box, 

pulley  end  of  shaft 2 

— Distance  from  end  of  yoke  arm  to  inner  end  of  journal  box, 

commutator  end  of  shaft 4 

e — Length  of  bushing I  ^ 

g — Length  of  journal  box. . ; 2\£ 

h — S  !ot  to  let  in  the  oil  ring HXI  ^ 

j — Outer  diameter  of  oil  reservoir 2 

Outer  length  of  reservoir,  axially l^ 

The  armature  'core  and  field  magnet  frames  may  be  wound  for  any 
voltage  desired,  but  the  most  efficient  windings,  as  the  cores  now  stand, 
will  be  those  specified  below. 

ARMATURE  WINDING. 

The  armature  core,  after  being  finally  assembled,  is  to  be  made 
ready  for  windings  by  applying  the  insulation.  Cut  out  four  discs  of 
heavy  canvas,  3  ins.  in  diameter,  with  a  ^-in.  hole  in  the  center;  varnish 
two  of  them  on  one  side  with  shellac  varnish,  and  apply  them  to  the  end 
plates  of  the  armature  core,  varnished  sides  in.  The  edges  will  turn 
over  to  cover  the  outer  edges  of  the  plates,  and  will  have  to  be  slitted 
at  intervals  of  y%  in.  all  around  to  prevent  bunching  up.  After  putting 
on  these  varnish  their  outer  faces,  and  one  face  of  each  of  the  remaining 
canvas  discs ;  when  the  varnish  begins  to  thicken  put  on  the  two  other 
discs,  one  at  each  end,  and  apply  considerable  pressure  to  them  until 
they  dry.  This  is  best  accomplished  by  boring  a  hole  in  a  piece  of  plank, 
large  enough  to  pass  the  shaft,  and  setting  the  core  on  the  plank,  on  end, 
next  putting  a  short  piece  of  board  (6  or  8  ins.  square)  with  a  hole  in  its 
center  on  the  upper  end  of  the  armature,  and  piling  any  convenient 
pieces  of  heavy  scrap  on  the  top  board. 

Next  insulate  the  slots  with  troughs  of  oil  paper,  1-64  in.  thick,  such 


ONE  HORSE-POWER   BIPOLAR  MOTOR  51 

as  is  used  with  the  ordinary,  office  outfit  for  copying  letters ;  each  trough 
should  consist  of  two  thicknesses  of  the  oil  paper,  and  the  floor  of  the 
trough  should  be  4%  ins.  long,  so  as  to  project  a  little  beyond  the  iron 
of  the  core  and  rest  upon  the  edges  of  the  canvas  discs,  which  were  previ- 
ously turned  over  to  cover  the  edges  of  the  end  plates. 

The  coils  may  then  be  wround  directly  in  the  slots,  each  coil  consist- 
ing of  twenty  turns  of  No.  18  wire,  four  wide  and  five  deep.  Each  slot 
will  contain,  when  the  windings  are  complete,  half  of  each  of  two  sepa- 
rate coils.  It  wrill  facilitate  the  winding  and  insure  electrical  balance  (as 
nearly  as  a  core  wound  armature  can  be  balanced)  if  the  builder  will 
make  a  diagram  of  his  armature  disc,  numbering  the  slots  from  I  to  24 
successively  around  the  circumference,  as  shown  by  Fig.  47.  Then  the 
winding  will  proceed  as  follows : 

First       coil       starts 

2d 

3d 

4th 

5th 

5th 

7th 

8th 

9th 
loth 
nth 

I2th 

---  '  i,  U 

~J<-n 

I4th 

I5th 

i6th 

i7th         "          " 

i 8th         "       .    " 

igth         "       '    " 

2oth 

2ISt  "  " 

22d 

23d 
24th 

After  winding  the  first  two  coils,  thin  strips  of  varnished  muslin 
should  be  laid  over  them,  across  each  armature  head  from  slot  to  slot,  so 
that  the  next  two  coils  will  be  insulated  from  the  first  pair;  each  suc- 
cessive pair  of  coils  should  receive  this  treatment,  and  after  the  slots  are 
half  filled  (twelve  coils  being  put  on),  a  strip  of  oil  paper  7-32  in.  wide 
and  4*4  ins.  long  must  be  laid  in  each  slot  on  top  of  the  coil  already  in 
place  before  proceeding  to  put  on  the  coil  which  next  goes  in  that  slot. 


No. 

No. 

in   slot    i  ^  ends 

in   slot   12 

13^  " 

"    »    24 

"    "    J7  ^  " 

4 

"    "    5V   " 

16 

;;  ;;  *£  ;; 

"     "     20 

8 

«   «    _   « 

14 

«<        «<        jgl       «C 

"     •«      2 

"    «    19  ^  « 

6 

«     <«      ,_  N    « 

18 

"     "     II*    " 

<«     "     22 

23 

10 

22 

"     "     II 

10 

23 

IS 

7 

**    "     6    " 

19 

14    " 

3 

«•     .«      2     t« 

15 

20 

««    »    9 

8 

«    <.    2I 

"  ./  "    16 

5 

4 

17 

24    " 

13 

it    «    I2    « 

"    "    i 

52  ELECTRICAL   DESIGNS 

The  starting  end  of  each  coil  should  l^e  kept  leading  out  straight 
from  its  slot,  and  the  finishing  end  should  be  brought  across  the  head 
and  secured  to  the  starting  end  by  a  turn  around  it.  When  the  winding 
is  complete,  untwist  the  finishing  end  of  each  coil  from'  its  starting  end, 
and  twist  it  and  the  starting  end  of  the  next  coil  to  the  right  firmly  to- 
gether. This  will  leave  twenty-four  terminals  to  lead  out  to  the  commu- 
tator lugs. 

Before  connecting  the  ends  to  the  commutator,  the  binding  wires 
should  be  put  on  and  the  winding  tested  for  grounds  on  the  core.  The 
binding  wires  are  put  on  in  two  bands,  and  consist  of  small  tinned  iron 
wire ;  they  should  be  put  on  beginning  i  in.  from  each  end  of  the  core, 
and  making  each  band  l/2  in.  wide.  The  binding  wire  should  be  wound 


FIG.    47. — WINDING   DIAGRAM. 


FIG.    48. — CONNECTING  DIAGRAM. 


on  strips  of  thin  varnished  muslin  laid  around  the  core  two  layers  deep, 
and  the  bands  should  be  soldered  at  four  equidistant  points  around  the 
armature  surface,  not  all  the  way  around.  The  wire  used  should  be  not 
larger  than  No.  22  B.  W.  G.  or  No.  20  B.  S.  G. 

Unless  the  machine  is  likely  to  be  used  in  very  dusty  surroundings 
it  is  better  not  to  put  any  covering  over  the  ends  of  the  armature  after  the 
winding  is  complete.  If  the  instructions-  for  insulating  each  pair  of  coils 
from  the  succeeding  pair  have  been  carefully  followed  out,  any  ordinary 
collection  of  dust  will  not  be  liable  to  cause  a  breakdown  in  the  heads. 
The  winding  just  described  is  intended  for  a  no-volt  machine.  If  it  is 
desirable  to  wind  the  armature  for  a  22O-volt  circuit,  use  No.  21  double 
cotton-covered  wire,  making  each  coil  five  turns  wide  and  six  layers 
deep. 


ONE  HORSE-POWER  BIPOLAR  MOTOR  53 

The  commutator  had  better  be  purchased  from  any  well-known 
manufacturer  of  commutators,  as  its  market  price  will  be  less  than  the 
cost  of  material  and  labor  necessary  to  make  one  properly.  It  must 
have  twenty-four  segments  and  be  not  more  than  2  ins.  long  along  the 
shaft ;  the  diameter  does  not  matter  particularly — take  one  of  a  stock  size 
from  the  maker.  In  connecting  up  the  coils  to  the  commutator  carry 
the  ends  previously  twisted  together  straight  out  to  the  commutator 
segments.  Fig.  48  shows  the  connections  diagrammatically.  The  slots 
are  omitted  and  each  coil  is  represented  as  having  only  one  turn  for  the 
sake  of  simplicity.  The  coils  are  lettered,  to  facilitate  identification  of 
opposite  ends.  The  ends  leading  straight  to  the  commutator  are  the 
starting  ends ;  those  leading  around  being  the  final  ends.  The  diagram 
is  not  intended  to  show  the  relative  radial  positions  of  the  coils,  and  care 
must  be  observed  to  avoid  becoming  confused.  For  example,  coil  A 
may  or  may  not  be  under  coil  Z  at  its  starting  side ;  they  are  both  in  the 
same  slot,  but  it  does  not  matter  which  is  on  top.  If  coil  A  was  the  first 
one  put  on,  it  will,  of  course,  be  in  the  bottom  of  both  of  its  slots,  and 
coil  Z  will  come  on  top  of  each  side  of  it.  The  diagram  only  shows  the 
relative  angular  positions  of  the  coils  and  the  manner  of  connecting  their 
ends.  The  brushes  should  be  of  carbon,  ^  in.  thick,  and  of  a  width  y$  in. 
less  than  the  length  of  the  commutator  face,  which  should  be  about  itf 
ins.  The  brush  holders  may  be  copied  from  any  standard  type  to  which 
the  builder  of  this  motor  has  access. 

The  iron-clad  magnet  requires  two  magnet  coils,  one  on  each  pole ; 
for  iio-volt  circuits  these  coils  consist  of  No.  22  wire  wound  in  6 1  layers, 
each  layer  being  50  turns  long.  If  both  the  coils  are  wound  in  the  same 
direction — in  other  words,  if  they  are  precisely  alike  as  to  the  manner  of 
winding,  as  they  should  be — the  beginning  end  of  one  must  be  connected 
to  the  final  end  of  the  other,  the  two  remaining  ends  being  carried  to  the 
terminals  of  the  machine.  The  best  arrangement  is  to  connect  the  two 
ends  that  are  farthest  apart,  making  this  connection  on  the  pulley  side  of 
the  machine.  For  22O-volt  circuits  the  wire  must  be  No.  25  gauge,  wound 
to  a  depth  of  75  layers,  65  turns  to  each  layer.  These  coils  should  be 
wound  on  a  block,  the  cross-section  of  which  is  of  exactly  the  same  shape 
as  that  of  the  magnet  core  at  its  largest  part,  but  which  measures  ^  in. 
more  in  each  direction.  After  winding  each  coil,  tie  it  at  each  corner 
with  coarse  linen  thread  (cobbler's  thread)  and  cover  it  with  strips  of 
muslin  wound  at  right  angles  to  the  direction  of  the  wires,  and  so  put  on 
as  to  have  the  edge  of  each  convolution  of  muslin  lay  just  alongside  that 
of  its  neighbor — touching  it  but  not  lapping  it.  The  muslin  must  be 
one-fourth  the  width  of  the  inner  edge  of  one  side  of  the  coil,  so  that 


54  ELECTRICAL  DESIGNS 

four  turns  will  cover  one  side  evenly.  Put  the  muslin  on  in  two  layers, 
the  turns  of  the  second  layer  covering  the  joint  between  the  turns  of  the 
first  layer.  Then  wind  strips  over  the  corners  cf  the  coils,  two  layers 
deep.  After  the  coil  is  covered  with  one  layer,  varnish  the  muslin  cover- 
ing heavily  with  shellac;  when  this  is  nearly  dry,  put  on  the  next  layer 
and  the  corner  strips,  and  after  varnishing  the  whole,  set  the  coil  aside  to 
dry.  Do  not  put  any  varnish  on  the  wire  itself.  Next  cover  the  iron 
cores  of  the  machine  with  a  layer  of  muslin,  this  time  lapping  the  edges 
cf  successive  convolutions ;  varnish  the  muslin,  and  when  it  and  the  coils 
are  thoroughly  dry.  put  the  latter  on.  Unless  the  pattern  for  the  field 
magnet  has  been  very  exactly  made,  and  the  casting  is  an  unusually 
perfect  one,  it  may  be  necessary  to  file  the  corners  of  the  pole  pieces 
slightly  to  get  the  coil  between  them  in  putting  on  the  magnet  core.  In 
filing  these  corners,  be  careful  to  round  them,  leaving  no  sharp  corners 
or  edges  whatever.  It  is  advisable  to  do  this,  even  if  it  is  not  mechan- 
ically necessary  for  the  introduction  of  the  coils. 

The  Jenny  type  of  magnet  has  only  one  magnet  coil,  which  consists 
of  No.  25  wire,  wound  to  a  depth  of  65  layers,  with  148  turns  to  a  layer, 
for  no-volt  service.  For  220  volts,  use  No.  28  wire,  wound  to  a  depth 
of  81  layers,  186  turns  to  each  layer.  All  the  wire  specified  herein  for 
both  armature  and  field  winding  should  be  double  cotton  covered.  Cir- 
cular magnet  heads  of  vulcanized  fibre  should  be  used  to  protect  the  ends 
of  the  coil,  as  the  full  voltage  of  the  machine  exists  between  these  ends ; 
these  heads  should  be  J^-in.  thick  and  63^  ins.  in  diameter,  with  a  hole 
to  fit  the  magnet  core  snugly  if  the  coil  is  wound  directly  on  the  core. 
If  not,  a  bobbin  should  be  made,  the  center  consisting  of  a  tube  of  1-32 
in.  fibre,  4^4  ins.  long,  and  of  an  internal  diameter  to  go  easily  over  the 
core ;  the  heads  of  the  bobbin  to  be  of  3/£-in.  fibre,  as  above.  If  the  coil 
is  wound  on  the  core,  the  latter  must  be  covered  with  three  layers  of 
muslin,  each  layer  varnished  with  shellac.  The  whole  must  dry  thor- 
oughly before  the  wire  is  wound  on. 

The  data  of  the  machine  are  as  follows : 

IRON-CLAD   TYPE. 

110  VoltS,  220  VoltS. 

Resistance  of  armature  winding I  ohm.  3.15  ohms, 

Armature  capacity,  maximum 8.3  amp.  4.7    amp. 

"         normal....' 7  amp.  3.8    amp. 

"         loss,  &R>  normal 49  watts.  45  K  watts. 

"            *'     hysteresis,  normal 11.7     "  13    " 

"            "     eddy  currents 1.3     "  1.5    '* 

Total  internal  armature  losses 62     "  60    " 

Magnetic  flux  per  sq.  in.  in  armature  core.  71,800  72,000 

Revolutions  per  minute,  loaded 1,800  2,000 


ONE  HORSE-POWER  BIPOLAR  MOTOR  55 

Resistance  of  field  winding 220  ohms.  682  ohms. 

Current  in  field  winding J^  amp.  0.322  amp. 

Heat  loss  in  field  winding 55  watts.  70.85    watts. 

Density  per  sq.  in.  in  core 38,000  38,160 

Density  per  sq.  in.  in  gaps 25,500  25,600 

Efficiency,  approximately 70^  75# 

SINGLE   COIL  TYPE. 

Armature  data  same  as  above.  no  volts.  220  volts. 

Resistance  of  field  coil 433  ohms.  1,400  ohms. 

Current  in  field  coil %  &mp.  0.16  amp. 

Heat  loss  in  field  coil 27^  watts.  35.2    watts. 

Density  per  sq.  in.  in  core 89,750  90,00 

Efficiency,  approximately 78$  79$ 

An  amateur  motor  builder  will  be  wise  not  to  attempt  to  make  a 
starting  box  for  this  size  of  machine ;  one  can  be  purchased  for  a  mod- 
erate sum  from  any  of  half  a  dozen  reputable  manufacturers,  and,  as 
either  of  the  motors  here  described  is  well  worth  the  outlay  necessary  to 
insure  its  protection  in  this  particular,  the  writer  advises  buying  the 
starting  rheostat. 

If,  however,,  the  reader  particularly  desires  to  make  his  own  starting 
rheostat,  the  arrangement  shown  by  Fig.  49  will  be  found  easier  to  con- 
struct than  anything  in  the  shape  of  a  wire  rheostat.  In  the  sketch,  L  is 
the  lever,  pivoted  on  a  J^-in.  metal  post,  and  normally  forced  down- 
wardly by  a  coil  spring  of  three  or  four  turns  (not  shown),  which  is 
located  under  the  washer,  W.  A  pin  through  the  post  secures  the 
washer,  spring  and  lever.  H  is  the  handle ;  a  wooden  handle,  such  as 
coffee  grinders  are  given,  or  a  large  porcelain  knob  will  answer.  B  is 
the  contact  brush  of  copper,  slitted  tangentially  to  the  circles  of  the  con- 
tact strips,  c,  c,  c,  c,  c,  c,  c\  these  circles  have 'their  common  center, 
of  course,  in  the  center  of  the  post  on  which  L  is  pivoted.  An  end  view 
of  the  brush,  B,  is  given  by  E,  showing  the  convex  shape  given  the 
under  face  of  the  brush  to  enable  it  to  pass  smoothly  over  the  contact 
strips.  The  end  of  these  should  be  beveled  to  avoid  digging  into  the 
brush. 

The  connections  are  shown  diagrammatically.  R  is  a  bank  of  five 
32  candle-power  incandescent  lamps,  rated  at  no  volts  (100  will  be 
better,  and  they  can  probably  be  readily  obtained) ;  C  is  the  motor  com- 
mutator; b,  b,  the  brushes;  F,  the  field  winding;  S,  a  double-pole  com- 
bined switch  and  fuse  block,  and  M,  the  service  mains.  A  glance  at  the 
connections  will  show  that  the  functions  of  the  lever  L  are  to  first  con- 
nect in  the  field,  next  the  armature  in  series  with  one  lamp ;  at  each  suc- 
cessive step  a  lamp  is  added  in  parallel  with  the  first  one  until  all  are  in, 


ELECTRICAL  DESIGNS 


and  the  last  position  of  the  lever  cuts  out  the  lamps,  leaving  the  armature 
in  circuit  direct. 

The  lamps  should  be  mounted  on  the  base  with  the  lever  and  con- 
tacts, and  it  is  preferable,  though  not  particularly  urgent,  that  the  switch 
S  be  mounted  on  that  base  also.  The  sketch  shows  the  lever  in  the  "off" 
position ;  the  switch,  S,  should  never  be  closed,  except  when  the  lever  L 
is  in  this  position.  If  the  reader  desires  to  make  the  arrangement  auto- 
matic he  need  only  add  a  retractile  spring  to  pull  the  lever  L  to  the  "off'* 


x^\       /^N/"^      X-* 

\O0OC 


FIG.   49. — MOTOR-STARTING  RHEOSTAT  AND  DIAGRAM  O?  CONNECTIONS. 

position ;  a  bar  of  iron  j£  in.  by  J4  in.  by  2  ins.  on  the  right  hand  edge  of 
the  lever,  and  a  small  magnet  connected  in  series  with  the  field,  F,  and 
located  on  the  base  about  Mg,  in  such  a  position  that  it  will  hold  the 
lever,  by  means  of  the  bar  of  iron,  when  it  is  brought  to  the  "on"  posi- 
tion. 


CHAPTER  VIT 


ONE  HORSE-POWER  FOUR-POLAR  MOTOR  WITH  DRUM  ARMATURE. 


For  the  four-polar  one  horse-power  motor  here  described  only  one 
type  of  field  magnet  is  shown,  namely,  the  familiar  ring  yoke  with  radial 
magnet  poles.  This  type  combines  more  good  points  than  any  other, 
hence  the  limitation  to  the  one  type.  A  choice  is  given,  however,  be- 
tween cast-iron  and  cast-steel.  The  armature  construction  is  the  same 


d-\" 


FIG.    50. 
DETAILS  OF  ARMATURE  CONSTRUCTION. 

for  both  types  of  field  magnet,  the  only  difference  being  in  the  length  of 
the  core  along  the  shaft,  and,  consequently,  the  length  of  the  shaft. 

Fig.  50  shows  the  shaft  and  a  cross-sectional  view  of  the  armature 
core.  The  discs  are  mounted  on  a  cast-iron  drum,  d,  which  has  a  flange, 
/,  and  a  hub,  h*,  at  one  end,  and  a  hub,  h,  at  the  other  end.  Fig.  5 1  gives 
a  transverse  cross-sectional  view  of  the  drum,  and  Fig.  52  is  a  perspective 


58  ELECTRICAL  DESIGNS 

view,  from  the  flangeless  end.  The  wall  of  the  drum  is  thickened  at  two 
places,  diametrically  opposite,  as  shown  in  Fig.  51.  This  is  necessary 
on  one  side  in  order  to  provide  sufficient  metal  under  the  key-seat ;  it  is 
necessary  on  the  opposite  side  to  obtain  a  mechanical  balance. 

The  discs  are  held  endwise  by  a  clamping  ring,  r,  which  may  be 
either  screwed  onto  the  end  of  the  drum,  d,  or  held  on  by  four  flat-headed 
screws  with  large  heads.  The  discs  are  held  from  turning  by  a  key.  At 
each  end  of  the  magnetic  core  a  disc  of  fibre,  indicated  by  heavy  black 
lines,  should  be  placed.  These  discs  must  be  exactly  like  the  iron  discs, 
except  that  they  are  1-16  inch  thick. 

The  iron  core  discs  are  5^  ins.  in  diameter  and  1-40  in.  thick,  with 
32  slots,  each  y±  m-  wide  and  9-16  in.  deep.  The  slots  have  parallel 
sides.  The  discs  must  be  of  the  best  charcoal  iron ;  the  hole  in  the  center 
is  3  ins.  in  diameter,  key-seated.  The  flange,  /,  and  the  clamping-ring, 
r,  must  have  their  outer  edges  rounded  off  to  avoid  cutting  the  insulation 
of  the  winding.  The  dimensions  of  the  core  drum  are  as  below : 

INCHES. 

Length  of  drum,  d 4^ 

Inner  diameter  of  d 2j£ 

Outer  diameter  of  d 3 

Diameter  of  flange,  f,  and  ring,  r 4^ 

Thickness  of  flange,  f ,  and  ring,  r. : }£ 

Thickness  of  d  at  thickest  point l/>. 

Diameter  of  hubs,  h  and  h2 ij£ 

Bore  of  hubs,  h  and  h3 i>£ 

Length  of  hub,  h ij^f 

Length  of  hub,  h3 I 

Length,  a,  of  disc  portion  of  core 4 

The  shaft  measurements  are  as  follows  : 

At  v  x  y  z 

Diameter,  inches %i  "fa  i/i  Y% 

Length,  inches 3  2l/2  7>^  6 

The  shoulders  where  v  and  x  meet  and  where  y  and  z  meet  should 
be  slightly  rounded  off  at  the  corner  and  filleted  in  the  angle.  A  key 
should  be  used  to  fasten  each  hub  to  the  shaft,  but  the  machine  will  doubt- 
less give  satisfaction  with  only  one  key,  that  one  being  in  the  hub,  h,  at 
the  pulley  end. 

Figs.  53  to  56  inclusive  show  end  and  side  views  and  cross-sections 
of  a  journal  pedestal  and  box.  The  two  bearings  are  alike  in  every  par- 
ticular, and  are  made  of  cast-iron.  The  base  or  foot  is  tooled  to  conform 
to  the  circle  to  which  the  pedestal  seat,  on  the  magnet  frame,  is  machined, 
and  is  j/2  in.  thick.  The  standard  or  pedestal  consists  of  two  ribs  at 
right  angles  to  each  other,  y2  in.  thick  and  having  curved  edges,  as 


ONE  HORSE-POWER   FOUR-POLAR  MOTOR 


59 


shown.  The  box  is  of  the  ring-oiling  type,  with  a  single  ring  hung  about 
midway  of  the  journal ;  the  bushing  is  easily  made  from  thin  brass  tubing, 
J/s  in.  outside  diameter,  and  with  a  very  thin  wall  (not  over  i~32d  in.), 
babbitted  to  fit  the  shaft  and  having  a  slot  y%  in.  wide  cut  half  way 
through  it,  nearly  midway  between  its  ends ;  accurately,  slot  must  be  J4 
in.  nearer  one  end  than  the  other.  The  bushing  is  2^/4  ins.  long;  the  oil 
ring  is  made  of  brass,  iJ/2  ins.  in  diameter  inside,  I  11-16  ins.  diameter 
outside,  and  $4  in.  wide  along  the  shaft.  Reference  to  the  side  views 
of  the  journal  pedestal  will  show  a  slot  in  the  upper  wall  of  the  box  por- 
tion, through  which  the  oil  ring  is  inserted  before  putting  in  the  bush- 
ing. A  cover  should  be  provided  for  this  slot  to  keep  out  dust,  etc. 
The  dimensions  of  the  journal  pedestals  are  as  follows  : 

INCHES. 

B — Radius  of  arc,  pedestal  seat $% 

g — Length  of  circular  oil  reservoir 2 

i — Length  of  journal  box 3 

Bore  of  journal  box ^ 

j — Diameter  of  oil  reservoir 2^ 

Internal  diameter  of  oil  reservoir 2 

J — Width  of  pedestal  foot 3 

k — Length  of  pedestal  foot 3 

The  bore  of  the  box  portion  of  the  pedestal  must,  of  course,  be  made 
to  fit  snugly  the  outer  diameter  of  the  tubing  used  for  a  bushing,  as  the 


r-  — J 

FIGS.    53,    54,    55,    56. — DETAILS   OF  JOURNAL  BOX    AND  PEDESTAL. 

wall  of  the  latter  is  too  thin  to  admit  of  turning  it  down  to  fit  a  prede- 
termined bore  in  the  pedestal.  After  boring  the  pedestal  to  fit  the  bush- 
ing it  should  be  mounted  on  a  mandrel  and  its  base  turned  to  the  radius 
By  of  55/2  ins.,  which  is  the  same  as  the  radius  of  the  circle  of  the  foot  on 


6o 


ELECTRICAL   DESIGNS 


WG,  50.— PLAN  VIEW  OF  FIELD-MAGNET;  EITHER  CAST-IRON  OR  CAST-STEEL. 


FIG.    6l. 
SHAPE  OF  MAGNET  CORE. 


FIG.   60. — SIDE  ELEVATION  OF  FIELD  MAGNET. 


LIBR 

OFTH1 


62  ELECTRICAL  DESIGNS 

the  magnet  frame.  Each  pedestal  should  be  fastened  to  the  foot  with 
four  %-in.  cap  screws. 

Of  the  two  field  mag-nets  shown,  the  cast-iron  one  will  be  found 
much  easier  to  make  because  there  is  less  tooling  to  be  done  and  iron 
castings  are  smoother  than  steel,  requiring  little  or  no  finishing  else- 
where than  the  pedestal  seats  and  pole  faces.  Fig  57  shows  the  cast-iron 
magnet  frame  and  Fig.  58  the  cast-steel  frame.  Fig.  59  is  a  plan  view 
of  either  frame  and  Fig.  60  is  an  edge  view. 

The  measurements  for  the  cast-iron  magnet  are  as  follows: 

INCHES. 

A — Bore  of  armature  chamber 5  ^ 

B — Radius  to  which  pedestal  seat  is  bored 5^2 

C — Outer  diameter  of  yoke  ring 13 

D — Distance  between  parallel  inner  faces  of  yoke  ring 10 

E — Width  of  plane  surface  behind  coil 5 

F — Width  of  magnet  coil 2}/z 

F2 — Breadth  of  magnet  core 4 

G — Distance  from  core  to  angle  of  yoke i^ 

H— Width  of  frame  foot 2 

Hg — Length  of  double  foot 4^ 

J — Width  of  pedestal  lug  and  seat 3 

K — Length  of  pedestal  lug  commutator  side 5^ 

L — Length  of  pedestal  lug  pulley  side 3^ 

k — Length  of  pedestal  seat 3 

M — Axial  width  of  magnet  yoke 7 

Fig.  6 1  shows  the  cross-section  of  a  magnet  core,  from  which  it  will 
be  seen  that  the  corners  of  the  core  are  rounded  off.  The  radius  of  the 
curve  here  is  %  in.  The  only  machining  that  should  be  required  for  this 
frame  is  boring  the  armature  chamber  and  pedestal  seats  and  drilling  12 
bolt-holes.  The  frame  should  be  clamped  to  a  lathe  carriage  with  its 
center  true  with  the  lathe  centers,  and  the  boring  done  at  one  setting  by 
means  of  a  boring  bar  and  tool.  Both  pedestal  seats  should  be  cut  be- 
fore the  frame  is  moved  from  its  original  position. 

The  magnet  must  be  made  of  the  very  best  grade  of  iron  obtainable ; 
use  Scotch  pig  if  possible.  It  should  be  allowed  to  remain  in  the  sand 
until  it  is  cold,  care  being  taken  not  to  remove  any  of  the  sand  around 
the  magnet  portion  until  the  casting  is  ready  to  come  out.  The  longer 
of  the  two  lugs  might  advantageously  be  placed  uppermost  in  putting  the 
pattern  in  the  sand,  and  after  the  casting  has  been  cooling  for  24  hours 
the  sand  may  be  scraped  away  from  the  end  of  this  lug  so  that  its  tem- 
perature may  be  noted  . 

The  steel  field  magnet  is  much  preferable  if  the  reader  has  the  skill 
and  facilities  to  make  it  properly.  The  difference  from  the  cast-iron 
magnet  consists  in  making  the  magnet  cores  round  instead  of  oblong,  anci 


O.V£  HORSE-POWER   POUR-POLAR  MOTOR 


putting  on  pole-shoes.  The  length  of  the  machine  is  thereby  reduced 
one  inch,  but  all  the  transverse  measurements  remain  unchanged.  The 
magnet  ends  arc  machined,  exactly  as  in  the  case  of  the  cast-iron  frame, 
but  the  bore,  A*t  is  greater,  namely,  6*/£  ins. 

The  pole-pieces  arc  made  in  one  piece,  called  a  polar-bushing,  like 
Fig.  62,  and  this  had  better  be  done  before  the  magnet  is  bored  out.  This 
bushing  is  a  simple  cylinder  of  cast-iron  with  four  openings  in  its  wall, 
equidistant  from  each  other.  Fig.  63  shows  the  exact  shape  of  each  of 
these  openings.  The  measurements  of  the  bushing  are  these : 

INCHES. 

A — Bore  of  bushing,  finished 5^$ 

A2 — Diameter  of  bushing,  finished 6y& 

a2 — Length  of  bushing,  finished 3  ^f 

b — Length  of  openings  in  wall 3 

c — Radius  of  curve,  side  of  opening 4 

e — Maximum  width  of  opening itf 

The  casting  for  this  bushing  should  be  about  3^  ins.  long,  6^  ins. 

in  diameter  and  5^  ins.  bore  in  the 
rough.  After  it  has  been  turned 
down  to  the  finished  diameter,  mount 
the  magnet  frame  and  bore  out  its 


K Q 


American  Electrician 


FIGS.   62  AND  63. — MAGNET  POLE  BUSHING.     _  ,     FIG.   64. — FIELD  COIL  CONNECTIONS. 

polar  circle  to  such  a  size  that  the  bushing  is  a  snug  fit — not  quite  a  driv- 
ing fit,  but  tight  enough  to  prevent  turning  by  hand.  Then  insert  the 
bushing  so  that  the  openings  in  its  sides  come  half  way  between  the 
magnet  cores,  and  scribe  the  outlines  of  two  opposite  cores  on  its  surface. 
Remove  the  bushing  and  set  a  steel  pin  at  each  extremity  of  each 


64 


ELECTRICAL  DESIGNS 


ellipse  scribed  on  the  surface.  Then  put  the  bushing  back  and  bore  it 
out  for  the  armature  chamber.  The  pins  will  take  up  against  the  edges 
of  the  magnet  cores  and  prevent  the  bushing  from  turning.  After  bor- 
ing it  out,  turn  on  the  ends  of  the  bushing  so  as  to  leave  the  connecting 
webs  from  pole  piece  to  pole-piece  %  in.  thick. 

The  objection  to  this  magnet  is  the  difficulty  of  fitting  the  bushing  to 
the  magnet  with  sufficient  accuracy  to  make  good  magnetic  contact  and 
still  leave  it  loose  enough  to  permit  removal  without  breaking  the  thin 
connecting  webs.  This  could  be  obviated  by  bolting  the  pole-pieces  to 
the  ends  of  the  magnet  cores  by  means  of  long,  slender  machine  screws, 
put  in  from  the  outside  of  the  yoke  through  holes  in  the  centers  of  the 
magnet  cores.  Then  the  connecting  webs  could  be  sawed  out  entirely, 
leaving  each  pole-shoe  independent  of  the  others.  This  construction  is 
also  magnetically  preferable,  and  if  the  builder  has  means  for  drilling  a 


FIG.   65. — CROSS    SECTION  OF  ARMATURE  SLOT,  SHOWING  ARRANGEMENT  OF    WIRES. 

J^-in.  hole  from  the  outside  of  the  ring  to  the  end  of  the  magnet  core  (a 
distance  of  3^  ins.),  the  pole-shoes  should  be  held  on  this  way. 

With  the  steel  magnet  the  following  measurements  must  be  substi- 
tuted for  those  previously  given : 

INCHES. 

F — Diameter  of  magnet  core 2^6 

M — Width  of  magnet  yoke 6 

a — Length  of  disc  part  of  core 3 

Length  of  drum,  d 3# 

Outer  diameter  of  drum,  d 2^ 

Inner  diameter  of  drum,  d 2j^ 

The  four  field  coils  for  the  cast-iron  magnet  frame  described  in  the 
preceding  chapter  are  of  No.  21  single-cotton-covered  magnet  wire.  The 
depth  of  the  winding  must  be  ij4  ms->  as  nearly  as  possible,  and  the 
length  along  the  core  should  be  2  ins.  Careful  and  close  winding  should 
give  40  layers  of  wire,  with  58  turns  to  a  layer.  Whatever  number  of 
turns  the  reader  may  obtain,  that  number  must  be  precisely  the  same  in 
?ill  four  coils.  In  order  to  attain  uniformity  the  coils  should  be  wound 
upon  a  frame  and  the  turns  religiously  counted. 

It  will  be  found  advantageous  to  tie  a  knot  in  the  starting  end  of 


ONE  HORSE-POll'ER   FOUR-POLAR  MOTOR 


each  coil  before  taping  it  so  that  it  may  be  identified  afterward.  The 
coils  must  be  connected  up  as  shown  by  the  diagram,  Fig.  64,  so  that  the 
starting  end  of  one  connects  to  the  finishing  end  of  its  neighbor.  This 
presupposes  that  all  four  are  wound  in  the  same  direction,  as  they 
should  be. 

The  coils  for  the  cast-steel  magnet  are  of  No.  24  single-cotton-cov- 
ered wire,  il/i  ins.  deep  and  i%  ins.  long.  Good  winding  will  enable 
the  reader  to  put  on  50  layers  of  wire  and  75  turns  to  a  layer.  As  in  the 
previous  case,  however,  the  depth  in  inches  is  the  essential  point,  though 
,it  is  advantageous  to  get  as  many  layers  in  that  depth  as  possible.  The 

coils  are,  of  course,  wound,  insulat- 

j      ed  and  connected  up  exactly  like  the 

I     oblong  coils  of  the  cast-iron  frame. 


t 


WIDH 


A. 


01,  m 


32 


FIG.    66. — DIMENSIONS   OF  ARMATURE  COIL, 


FIG.   67. — WINDING  DIAGRAM. 


The  armature  core  for  either  of  the  magnet  frames  will  contain  32 
coils ;  each  coil  consists  of  No.  21  double-cotton-covered  wire,  wound  five 
turns  wide  by  four  layers  deep.  Each  slot  contains  one  side  of  each  cf 
two  coils,  so  that  the  cross-section  of  the  winding  in  a  slot  will  be  as  in 
Fig.  65,  except  that  the  wire  will  lie  closer  together  than  the  sketch  indi- 
cates. All  armature  coils  should  be  wound  on  a  forming  bobbin  so  that 
they  will  all  be  exactly  alike.  Fig.  66  shows  what  the  essential  dimen- 
sions should  be.  The  width  of  the  hollow  of  the  coil  is  the  same  for 
both  armature  cores.  As  the  armature  core  to  be  used  with  the  steel 
magnet  is  an  inch  shorter  than  the  other  one,  the  coils  for  this  core  must 
be  an  inch  shorter ;  hence  the  two  dimensions  for  coil  lengths. 

Fig.  67  is  a  winding  diagram  and  shows  the  first  four  coils  in  posi- 
tion. The  coils  are  indicated  by  a  single  line  across  the  head  and  doit 


66 


ELECTRICAL  DESIGNS 


in  the  slots  for  simplicity.  The  builder  should  note  that  the  left-hand  side 
of  each  coil  is  in  the  bottom  of  the  slot  and  the  right-hand  side  is  on  top ; 
this  should  be  true  of  every  coil.  The  starting  ends  should  be 
knotted  for  identification,  and  all  the  knotted  ends  should  occupy  the 
same  relative  position  on  the  core.  For  smoothness  of  finished  heads 
the  coils  should  be  put  on  the  core  in  the  following  order: 

Coils    i,     2,  3,  4  in  Slots  I,     9,  17,  25. 

Coils    5,     6,  7,  8  in  Slots  2,  10,  18,  26. 

Coils    9,   10,  ii,  12  in  Slots  3,  n,  19,  27. 

L                           Coils  13,  14,  15,  16  in  Slots  4,  12,  20,  28. 

Coils  17,  18,  19,  20  in  Slots  5,   13,  21,  29. 

Coils  21,  22,  23,  24  in  Slots  6,  14,  22,  30. 

Coils  25,  26,  27,  28  in  Slots  7,  15,  23,  31. 

Coils  29,  30,  31,  32  in  Slots  8,  16,  24,  32. 

If  put  in  properly,  the  coils  will  give  a  regular  sequence  of  knotted 
ends  and  straight  ends,  one  each  projecting  from  each  slot.     The  con- 


TERMINAL 


FIG.    68. — COMMUTATOR  DIAGRAM. 


FIG.   69.— CONNECTIONS  BETWEEN 
BRUSHES. 


nections  to  the  commutator  are  then  simple.  Carry  all  the  knotted  ends 
straight  out  to  the  commutator,  and  the  straight  ends  one  segment  less 
than  a  quarter  circle  backwards  from  their  corresponding  knotted  ends. 
Thus,  the  knotted  end  from  slot  No.  i  goes  to  commutator  segment  No. 
i  (see  Fig  68),  and  so  on,  all  the  way  around.  Then  the  straight  ends 
go  to  the  commutator  as  follows : 

From  Slot  No.     i  to  Segment  No.  26. 

From  Slot  No.     2  to  Segment  No.  27. 

.    From  Slot  No.     3  to  Segment  No.  28. 


ONE  HORSE-POWER   FOUR-POLAR  MOTOR  67 

From  Slot  No.  4  to  Segment  No.  29. 
From  Slot  No.  5  to  Segment  No.  30. 
From  Slot  No.  6  to  Segment  No.  31. 
From  Slot  No.  7  to  Segment  No.  32. 
From  Slot  No.  8  to  Segment  No.  I. 
From  Slot  No.  9  to  Segment  No.  2. 
From  Slot  No.  10  to  Segment  No.  3. 
From  Slot  No.  u  to  Segment  No.  4. 
From  Slot  No.  12  to  Segment  No.  5. 
From  Slot  No.  13  to  Segment  No.  6. 
From  Slot  No.  14  to  Segment  No.  7. 
From  Slot  No.  15  to  Segment  No.  8. 
From  Slot  No.  16  to  Segment  No.  9. 
From  Slot  No.  17  to  Segment  No.  10. 
From  Slot  No.  18  to  Segment  No.  n. 
From  Slot  No.  19  to  Segment  No.  12. 
From  Slot  No.  20  to  Segment  No.  13. 
From  Slot  No.  21  to  Segment  No.  14. 
From  Slot  No.  22  to  Segment  No.  15. 
From  Slot  No.  23  to  Segment  No.  16. 
From  Slot  No.  24  to  Segment  No.  17. 
From  Slot  No.  25  to  Segment  No.  18. 
From  Slot  No.  26  to  Segment  No.  19. 
Frcm  Slot  No.  27  to  Segment  No.  20. 
From  Slot  No.  28  to  Segment  No.  21. 
From  Slot  No.  29  to  Segment  No.  22. 
From  Slot  No.  30  to  Segment  No.  23. 
From  Slot  No.  31  to  Segment  No.  24. 
From  Slot  No.  32  to  Segment  No.  25. 

The  commutator  must  have  32  segments,  as  indicated  by  Fig.  68, 
and  should  be  purchased  already  built  for  assured  satisfaction.  The 
brush  surface  of  the  commutator  must  be  i%  ins.  long",  at  least,  so  that 
carbon  brushes  i  in.  wide  and  J4  in.  thick  can  be  used.  The  diameter 
of  the  barrel  of  the  commutator  should  be  not  less  than  3,  and  preferably 
4  ins.  The  brush  holders  and  yoke  may  be  copied  advantageously  from 
any  of  the  standard  machines  now  on  the  market.  Four  brushes  must 
be  used,  and  the  two  diametrically  opposite  are  connected  together,  as 
shown  by  Fig.  69. 

The  windings  just  described  are  for  machines  to  work  on  a  110-115- 
volt  circuit.  If  windings  for  220-230  volts  are  desired  the  armature  coils 
should  be  of  No.  25  wire,  each  coil  five  layers  deep  and  eight  turns  wide, 
making  ten  layers  of  wire  per  slot.  The  field  coils  of  the  cast-iron  mag- 
net must  be  of  No.  24  s.c.c.  wire,  wound  to  the  dimensions  specified 
above,  namely,  \Y\  ins.  deep  and  2  ins.  long.  The  coils  for  the  cast-steel 
magnet  will  be  of  No.  27  wire  wound  to  a  depth  of  i1/^  ins.  and  a  length 
18  ins. 


68 


ELECTRICAL  DESIGNS 


The  principal  magnetic  and  electrical  data  of  the  two  machines  are 
below  : 


II5-VOLT  MOTOR. 


Cast-iron. 
I  ohm 
9  amp. 
81  watts 
4.1A  watts 


Resistance  armature  winding 

Normal  armature  currents 

C6R  loss  armature 

Hysteresis  and  eddy  currents , 

Approximate  speed 1600 

Resistance  field  winding 183  ohms 

Normal  field  current 628  amp. 

C8  R  field  loss 72.22  watts 

Flux  per  pole • 

Density  in  field  cores 48,000 

Density  in  air  gap 27,500 

Efficiency,  assuming  10  per  ) 
cent  friction  and  windage  \ 


75  percent 


Cast-steel. 
0.9 

9  amp. 
73  watts 
3  watts 
1600 

424  ohms 
.27  amp. 
31  watts 
400,000  lines 
93,000 
37,500 

80  per  cent 


As  in  the  preceding  case,  it  is  by  far  preferable  to  buy  a  starting  box 
from  one  of  the  standard  rheostat  manufacturers.  If  the  reader  insists 
upon  having  a  home-made  one,  however,  the  arrangement  shown  by  Fig. 
49  and  described  on  pages  55  and  56  will  answer. 


CHAPTER  VIII. 


TWO  HORSE-POWER    FOUR-POLAR    MOTOR  WITH  TWO-PATH  DRUM 

ARMATURE. 


For  this  motor,  as  in  the  preceding  design,  only  one  type  of  field 
magnet  is  shown,  namely,  the  familiar  ring  yoke  with  radial  magnet 
poles ;  a  choice  is  given  between  cast-iron  and  cast-steel  field  magnets. 
The  armature  construction  is  identical  for  both  types  of  magnet,  there 
being  a  difference  only  in  the  length  of  the  armature  and  shaft. 

Fig.  70  shows  the  shaft  and  a  cross-sectional  view  of  the  armature 
core.  The  discs  are  mounted  on  a  cast-iron  drum,  d,  which  has  a  flange, 
/,  and  a  hub,  Ii2}  at  one  end,  and  a  hub,  h,  at  the  other  end.  Fig.  71  gives 


FIG.  70.— ARMATURE  SHAFT  AND  CROSS-SECTION  OF  ARMATURE  CORE. 

a  transverse  cross-sectional  view  of  the  drum,  and  Pig.  72  is  a  perspective 
view,  from  the  flangeless  end.  The  wall  of  the  drum  is  thickened  at  two 
places,  diametrically  opposite,  as  shown  in  Fig.  71.  This  is  necessary  on 
one  side  in  order  to  provide  sufficient  metal  under  the  key-seat;  it  is 
necessary  on  the  opposite  side  to  obtain  a  mechanical  balance. 

The  discs  are  held  endwise  by  a  clamping  ring,  r,  which  may  be 
either  screwed  onto  the  end  of  the  drum,  d,  or  held  on  by  four  flat-headed 
screws  with  large  heads.  The  discs  are  held  from  turning  by  a  key.  At 
each  end  of  the  magnetic  core  a  disc  of  fibre,  indicated  by  heavy  black 
lines,  should  be  placed.  These  discs  must  be  exactly  like  the  iron  discs, 
Lexcept  that  they  are  1-16  in.  thick. 


ELECTRICAL   DESIGNS 


The  iron  core  discs  are  6]4  ins.  outside  diameter  and  1-40  in.  thick, 
with  43  slots,  each  ]/[  in.  wide  and  9-16  in.  deep.  The  slots  have  parallel 
sides.  The  discs  must  be  of  the  best  charcoal  iron ;  the  hole  in  the  center 
is  35^  ins.  in  diameter,  key-seated.  The  flange,  /,  and  the  clamping- 
ring,  r,  must  have  their  outer  edges  rounded  off  to  avoid  cutting  the  in- 
sulation of  the  winding.  The  dimensions  of  the  core  drum  are  as  below : 

Length  of  drum,  d 5^ 

Inner  diameter  of  d 3 

Outer  diameter  of  d ^H 

Diameter  of  flange,  f,  and  ring,  r.,  5^4 
Thickness  of  flange,  f,  and  ring,  r.  •£$ 
Thickness  of  d  at  thickest  point. . .  Y2 

Diameter  of  hubs,  h  and  h2 2 

Bore  of  hubs,  h  and  h2 itf 

Length  of  hub,  h 1*4 

Length  of  hub,  h8 l*^ 

Length  of  a,  of  disc  portion  of  core  4^ 


FIGS.    71   AND   72. 
CORE  DRUM  AND   HEAR. 


The  shaft  measurements  are  as  follows  : 

At    v  x  y  z 

Diameter,  inches %{  I  ij£  %( 

Length,  inches 3^$  3  9  7^ 

The  shoulders  where  v  and  x  meet  and  where  y  and  2  meet  should 
be  slightly  rounded  off  at  the  corner  and  filleted  in  the  angle.    A  key 


it- — . — * 


r d— 

FIGS.    73,    74,   75,   76.— DETAILS  ©F  JOURNAL  BOX. AND  PEDESTAL. 

should  be  used  to  fasten  each  hub  to  the  shaft,  but  the  machine  will  doubt- 
less give  satisfaction  with  only  one  key,  that  one  being  in  the  hub,  A,  at 
the  pulley  end.  The  hub,  h,  must  be  exactly  %  'm-  fr°m  tne  shoulder 
on  the  shaft. 

Figs.  73  to  76j  inclusive,  show  end  and  side  views  and  cross-sections 


TWO  HORSE-POWER   FOUR-POLAR  MOTOR  JI 

of  a  journal  pedestal  and  box.  The  two  bearings  are  alike  in  every  par- 
ticular, and  are  made  of  cast-iron.  The  base  or  foot  is  tooled  to  con- 
form to  the  circle  to  which  the  pedestal  seat,  on  the  magnet  frame,  is 
machined,  and  is  %  in.  thick.  The  standard  or  pedestal  consists  of  two 
ribs  at  right  angles  to  each  other,  %  in.  thick  and  having  curved  edges, 
as  shown.  The  box  is  of  the  ring-oiling  type,  with  a  single  ring  hung 
cbout  midway  of  the  journal ;  the  bushing  is  easily  made  from  thin  brass 
tubing,  I  in.  outside  diameter,  and  with  a  very  thin  wall  (not  over  i~32d 
in.),  babbitted  to  fit  the  shaft  and  having  a  slot  7-16  in.  wide  cut  half  way 
through  it,  nearly  midway  between  its  ends ;  accurately,  the  slot  must  be 
J4  in.  nearer  one  end  than  the  other.  The  bushing  is  3^  ins.  long;  the 
oil  ring  is  made  of  brass,  2  ins.  in  diameter  inside,  2*4  ins.  diameter  out- 
side, and  Y§  in.  wide  along  the  shaft.  Reference  to  the  side  views  of  the 
journal  pedestal  will  show  a  slot  in  the  upper  wall  of  the  box  portion, 
through  which  the  oil  ring  is  inserted  before  putting  in  the  bushing.  A 
cover  should  be  provided  for  this  slot  to  keep  out  dust,  etc.  The  dimen- 
sions of  the  journal  pedestals  are  as  follows : 

INCHES. 

B — Radius  of  arc,  pedestal  seat 6 

g — Length  of  circular  oil  reservoir 2 

i — Length  of  journal  box 3f£ 

Bore  of  journal  box I 

j — Diameter  of  oil  reservoir 2j£ 

Internal  diameter  of  oil  reservoir z% 

J— Width  of  pedestal  foot 3^ 

k — Length  of  pedestal  foot 3)6 

The  bore  cf  the  box  portion  of  the  pedestal  must,  of  course,  be  made 
to  fit  snugly  the  outer  diameter  of  the  tubing  used  for  a  bushing,  as  the 
wall  of  the  latter  is  too  thin  to  admit  of  turning  it  down  to  fit  a  prede- 
termined bore  in  the  pedestal.  After  boring  the  pedestal  to  fit  the  bush- 
ing it  should  be  mounted  on  a  mandrel  and  its  base  turned  to  the  radius 
B,  of  6  ins.,  which  is  the  same  as  the  radius  of  the  circle  of  the  foot  on  the 
magnet  frame.  Each  pedestal  should  be  fastened  to  the  foot  with  four 
5-16  in.  cap  screws. 

Of  the  two  field  magnets  shown,  the  cast-iron  machine  *^vill  be  found 
easier  to  make  because  there  is  less  tooling  to  be  done  and  iron  castings 
are  smoother  than  steel,  requiring  little  or  no  finishing  elsewhere  than 
the  pedestal  seats  and  pole  faces.  Fig.  77  shows  the  cast-iron  magnet 
frame  and  Fig.  78  the  cast-steel  frame.  Fig.  79  is  a  plan  view  of  either 
frame  and  Fig.  80  is  an  edge  view. 


ELECTRICAL  DESIGNS 


TWO  HORSE-POWER   FOUR-POLAR  MOTOR 


73 


FIG.  79.— PLAN  VIEW  OF  FIELD  MAGNET;   EITHER  CAST-IRON  OR  CAST-STEEL, 


— Fa— 

FIG.  8 I. 
SHAPE  OF  MAGNET  POLE. 


d. 


f  ivr 

-— H;- 


FIG.   8O. — SIDE  ELEVATION  OF  FIELD  MAGNET. 


74  ELECTRICAL   DESIGNS 

The  measurements  for  the  cast-iron  magnet  are  as  follows : 

INCHES. 

A — Bore  of  armature  chamber 7 

B — Radius  to  which  pedestal  seat  is  bored 6)£ 

C — Outer  diameter  of  yoke  ring 15^ 

D — Distance  between  parallel  inner  faces  of  yoke  ring 12 

E — Width  of  plane  surface  behind  coil 6 

T — Width  of  magnet  core 3 

F8 — Breadth  of  magnet  core 4>£ 

G — Distance  from  core  to  angle  of  yoke i^ 

H— Width  of  frame  foot 2)4 

H8 — Length  of  double  foot 5 

J  —Width  of  pedestal  lug  and  scat 3^ 

K — Length  of  pedestal  lug;  commutator  side 6j£ 

L — Length  of  pedestal  lug;  pulley  side 3^ 

K — Length  of  pedestal  scat 3^ 

M — Axial  width  of  magnet  yoke 8/75- 

Fig.  8 1  shows  the  cross-section  of  a  magnet  core,  from  which  it  will 
be  seen  that  the  corners  of  the  core  are  rounded  off.  The  radius  of  the 
curve  here  is  0.3  in.  The  only  machining  that  should  be  required  for 
this  frame  is  boring  the  armature  chamber  and  pedestal  seats  and  drill- 
ing 12  bolt-holes.  The  frame  should  be  clamped  to  a  lathe  carriage  with 
its  center  true  with  the  lathe  centers,  and  the  boring  done  at  one  setting 
by  means  of  a  boring  bar  and  tool.  Both  pedestal  seats  should  be  cut 
before  the  frame  is  moved  from  its  original  position. 

The  magnet  must  be  made  of  the  very  best  grade  of  iron  obtainable ; 
use  Scotch  pig  if  possible.  It  should  be  allowed  to  remain  in  the  sand 
until  it  is  cold,  care  being  taken  not  to  remove  any  of  the  sand  around 
the  magnet  portion  until  the  casting  is  ready  to  come  out.  The  longer 
cf  the  two  lugs  might  advantageously  be  placed  uppermost  in  putting  the 
pattern  in  the  sand,  and  after  the  casting  has  been  cooling  for  24  hours 
the  sand  may  be  scraped  away  from  the  end  of  this  lug  so  that  its  tem- 
perature may  be  noted. 

The  cast-steel  field  magnet  is  much  preferable  if  the  reader  has  the 
'".kill  and  facilities  to  make  it  properly.  The  difference  from  the  cast- 
_ron  magnet  consists  in  making  the  magnet  cores  round  instead  of  ob- 
long, and  putting  on  pole-shoes.  The  length  of  the  machine  is  thereby 
reduced  ij4  ms->  but  all  the  transverse  measurements  remain  un- 
changed. The  magnet  ends  are  machined  exactly  as  in  the  case  of  the 
cast-iron  frame,  but  the  bore,  A2,  is  greater,  namely,  /J/2  ins. 

The  pole-pieces  are  made  in  one  piece,  called  a  polar-bushing,  like 
Fig.  02,  and  this  had  better  be  done  before  the  magnet  is  bored  out. 
!This  bushing  is  a  simple  cylinder  of  cast-iron  with  four  openings  in  its 


TWO   HORSE-POWER   FOUR-POLAR  MOTOR 


75 


wall,  equidistant  from  each  other.     Fig.  83  shows  the  exact  shape  of  each 
of  these  openings.     The  measurements  of  the  bushing  are  these : 

INCHES. 

A8 — Diameter  of  bushing,  finished 7/^ 

A — Bore  of  bushing,  finished 7 

as — Length  of  bushing,  finished 3^ 

b — Length  of  openings  in  walls , 3/2 

c — Radius  of  curve,  side  of  opening 4j£ 

e — Maximum  width  of  opening 2>£ 

The  casting  for  this  bushing  should  be  about  4  ins.  long,  7^  ins.  in 
diameter  and  6^J  ins.  bore,  in  ihe  rough.  After  it  has  been  turned 
down  to  the  finished  diameter,  mount  the  magnet  frame  and  bore  out  its 
polar  circle  to  such  a  size  that  the  bushing  is  a  snug  fit — not  quite  a  driv- 


K — e — -w 

FIGS.   82  AND  83.— MAGNET  POLE  BUSHING.  FIG.    84.—FIELD  COIL  CONNECTIONS. 

ing  fit,  but  tight  enough  to  prevent  turning  by  hand.  Then  insert  the 
bushing  so  that  the  openings  in  its  sides  come  half  way  between  the  mag- 
net cores,  and  scribe  the  outlines  of  two  opposite  cores  on  its  surface. 

Remove  the  bushing  and  set  a  steel  pin  at  each  extremity  of  each 
ellipse  scribed  on  the  surface.  Then  put  the  bushing  back  and  bore  it 
out  for  the  armature  chamber.  The  pins  will  take  up  against  the  edges 
of  the  magnet  cores  and  prevent  the  bushing  from  turning.  After  bor- 
ing it  out,  turn  off  the  ends  of  the  bushing  so  as  to  leave  the  connecting 
webs  from  pole-piece  to  pole-piece  ]/%  in.  thick,  measured  axially. 

The  objection  to  this  magnet  is  the  difficulty  of  fitting  the  bushing 
to  the  magnet  with  sufficient  accuracy  to  make  good  magnetic  contact 


76  ELECTRICAL  DESIGNS 

and  still  leave  it  loose  enough  to  permit  removal  without  breaking  the 
thin  connecting  webs.  This  could  be  obviated  by  bolting  the  pole-pieces 
to  the  ends  of  the  magnet  cores  by  means  of  long,  slender  machine 
screws,  put  in  from  the  outside  of  the  yoke  through  holes  in  the  centers 
of  the  magnet  cores.  Then  the  connecting  webs  could  be  sawed  out 
entirely,  leaving  each  pole-shoe  independent  of  the  others.  This  con- 
struction is  also  magnetically  preferable,  and  if  the  builder  has  means 
for  drilling  V^-m.  holes  through  the  magnet  cores  from  the  outside  of  the 
yoke  ring  to  the  inside  of  the  bushing  (a  distance  of  4^  ins.),  the  pole- 
shoes  should  be  held  on  this  way. 

With  the  steel  magnet  the  following  measurements  must  be  substi- 
tuted for  those  previously  given : 

INCHES. 

F — Diameter  of  magnet  core 27/& 

M — Width  of  magnet  yoke 7Tss 

a — Length  of  disc  part  of  core 3>£ 

Length  of  drum,  d 3j| 

Length  of  y,  on  shaft 7^ 

This  is  to  say,  the  machine  must  be  exactly  1^4  ins.  shorter,  axially. 

The  four  field  coils  for  the  cast-iron  magnet  frame  are  of  No.  22 
single-cotton-coverecl  magnet  wire.  The  depth  of  the  winding  must  be 
IjX  ins.,  as  nearly,  as  possible,  and  the  length  along  the  core  should  be 
2*4  ins.  Careful  and  close  winding  should  give  50  layers  of  wire,  with 
70  turns  to  a  layer.  Whatever  number  of  turns  the  reader  may  obtain, 
that  number  must  be  precisely  the  same  in  all  four  coils.  In  order  to 
attain  uniformity  the  coils  should  be  wound  upon  a  frame  and  the  turns 
religiously  counted. 

It  will  be  found  advantageous  to  tie  a  knot  in  the  starting  end  of 
each  coil  before  taping  it  so  that  it  may  be  identified  afterward.  The 
coils  must  be  connected  up  as  shown  by  the  diagram,  Fig.  84,  so  that 
the  starting  end  of  one  connects  to  the  finishing  end  of  its  neighbor. 
This  presuppose!  that  all  four  are  wound  in  the  same  direction,  as  they 
should  be. 

The  coils  for  the  cast-steel  magnet  are  of  No.  23  single-cotton- 
covefed  wire,  1^2  ins.  deep  and  2  ins.  long.  Good  winding  will  enable 
the  reader  to  put  on  56  layers  of  wire  and  70  turns  to  a  layer.  As  in  the 
previous  case,  however,  the  depth  in  inches  is  the  essential  point,  though 
it  is  advantageous  to  get  as  many  layers  in  that  depth  as  possible.  The 
coils  are,  of  course,  wound,  insulated  and  connected  up  exactly  like  the 
oblong  coils  of  the  cast-iron  frame. 

The  armature  core  for  either  of  the  magnet  frames  will  contain  43 


r:ro  HORSE-POWER  FOUR-POLAR  MOTOR 


77' 


coils;  each  coil  consists  of  No.  16  double-cotton-covered  wire,  wound 
three  turns  wide  by  three  layers  deep.  Each  slot  contains  one  side  of 
each  of  two  coils,  so  that  the  cross-section  of  the  winding  in  a  slot  will  be 
as  in  Fig.  85. 

All  armature  coils  should  be  wound  on  a  forming  bobbin  so  that  they 


FIG.    85. — CROSS-SECTION  OF  A  SLOT. 


FIG.    88. — CONNECTING  DIAGRAM. 


will  all  be  exactly  alike.  Fig.  86  shows  what  the  essential  dimensions 
should  be.  The  width  of  the  hollow  of  the  coil  is  the  same  for  both 
armature  cores.  As  the  armature  core  to  be  used  with  the  steel  magnet 
is  ij4  ins.  shorter  than  the  other  one,  the  coils  for  this  core  must  be  cor- 
respondingly'shorter ;  hence  the  two  dimensions  for  coil  lengths* 

Fig.  87  is  a  winding  diagram  and  shows  four  coils  in  position.    The 
coils  are  indicated  by  single  lines  across  the  head  and  dots  in  the  slots  for 


_     A NGO'LAR  SPREAD 


FIG.  86. — DIMENSIONS  OF  ARMATURE  COIL. 


FIG.   87. — WINDING  DIAGRAM. 


simplicity.  The  builder  should  note  that  the  left-hand  side  of  each  coil 
is  in  the  bottom  of  the  slot  and  the  right-hand  side  is  on  top ;  this  should 
be  true  of  every  coil,  but  it  is  not  imperative.  The  machine  will  work 
just  as  well  if  half  of  the  coils  are  put  on  with  both  sides  bottom  and  the 
other  half  on  top  of  them,  but  the  job  will  not  be  so  neat  on  the  armature 


78  ELECTRICAL  DESIGNS 

heads.  The  spacing  or  pitch  of  the  coils  must  be  exactly  as  indicated — 
IO  slots  in  between  the  two  sides  of  each  coil. 

The  starting  ends  should  be  knotted  for  identification,  and  all  the 
knotted  ends  should  occupy  the  same  relative  position  on  the  core.  If 
put  in  properly,  the  coils  will  give  a  regular  sequence  of  knotted  ends  and 
straight  ends,  one  each  projecting  from  each  slot.  The  connections  to 
the  commutator  are  then  simple.  Carry  all  the  knotted  ends  straight  out 
•  to  the  commutator,  and  each  straight  end  to  a  segment  22  bars  from  the 
one  to  which  the  knotted  end  is  connected.  Fig.  88  represents  one  coil 
connected,  and  shows  that  there  are  21  segments  between  the  two  to 
which  the  coil  ends  go,  reckoning  around  that  side  of  the  commutator 
nearest  the  coil  itself.  This  spacing  must  be  observed  throughout. 

The  commutator  must  have  43  segments  and  should  be  purchased 
already  built  for  assured  satisfaction.  The  brush  surface  of  the  commu- 
tator must  be  i^  ins.  long,  at  least,  so  that  carbon  brushes  i }/\  ins.  wide 
and  2/8  ins.  thick  can  be  used.  The  diameter  of  the  barrel  of  the  commu- 
tator should  not  be  less  than  4,  and  preferably  5  ins.  The  brush  holders 
and  yoke  may  be  copied  advantageously  from  any  of  the*  standard  ma- 
chines now  on  the  market.  Only  two  brushes  are  to  be  used,  and  these 
set  precisely  a  quarter  of  a  circle  apart,  reckoning  around  the  barrel  of 
the  commutator.  •_ 

The  windings  just  described  are  for  machines  to  work  on  a  110-115- 
volt  circuit.  If  windings  for  220-230  volts  are  desired  the  armature  coils 
should  be  of  No.  19  wire,  each  coil  five  layers  deep  and  four  turns  wide, 
making  ten  layers  of  wire  per  slot.  The  field  coils  of  the  cast-iron  mag- 
net must  be  of  No.  26  s.c.c.  wire,  wound  to  the  dimensions  specified 
above,  namely,  ij/2  ins.  deep  and  2j4  ins.  long.  The  coils  for  the  cast- 
steel  magnet  will  be  of  No.  27  wire  wound  to  a  depth  of  ij^>  ins.  and  a 
length  of  2  ins.  For  5oo-volt  service  use  No.  23  double-cotton-covered 
wire  on  the  armature,  six  turns  wide  and  seven  layers  deep,  per  coil ;  84 
wires  per  slot.  On  the  cast-iron  magnet  use  No.  29  double-covered 
wire  and  on  the  steel  magnet  No.  30. 

The  principal  magnetic  and  electrical  data  of  the  two  machines  are 
below : 

II5-VOLT   MOTOR. 

Cast-iron.  Cast-Steel. 

Resistance  armature  winding I  ohm  0.9 

Normal  armature  current 18  amp.  18  amp. 

Approximate  speed 1325  r.  p.  m.  1325  r.  p.  m. 

Flux  per  pole 340,000  lines 

Density  in  field  cores 33, 700  70,000 

Density  in  air  gap 26,000  38,000 


CHAPTER  IX. 


THREE  HORSE-POWER  MOTOR. 


The  motor  design  which  forms  the  subject  of  this  chapter,  although 
somewhat  similar  to  those  described  in  Chapters  VII  and  VIII,  differs 
considerably  in  the  constructional  details  of  the  magnet.  Here  a  cast- 
iron  ring  and  wrought  iron  cores  are  employed  with  a  view  to  simplifying 
the  work  as  far  as  possible  without  sacrificing  the  efficiency  of  the 
machine,  and  also  without  making  it  unduly  heavy.  The  cast-iron  ring 
is  preferably  made  in  a  single  piece  and  the  wrought-iron  cores  are 
turned  to  a  very  slight  taper  and  drawn  into  holes  in  the  yoke  ring  by 
means  of  a  bolt  and  heavy  washer  from  the  outside.  Unless  the  builder 
has  excellent  machine-shop  facilities,  however,  and  is  an  expert  machinist,, 
this  construction  will  be  found  rather  difficult,  as  it  is  necessary  to  have  a 
perfect  fit  between  the  taper  of  the  magnet  core  and  that  of  the  hole  in 
which  it  is  seated. 

As  an  alternative  the  magnet  frame  can  be  cast  in  two  pieces,  the 
division  being  along  the  line,  x,  Fig.  91.  If  the  motor  is  built  in  this 
way,  each  half  must  be  chucked  and  the  joint  faced  off  fairly  smooth,  al- 
though it  is  not  necessary  to  have  a  perfect  joint,  as  no  magnetic  lines  of 
force  cross  the  break.  After  truing  up  the  abutting  facer  of  each  half  of 
the  magnet  ring  the  two  halves  should  be  clamped  together  with  i-32-in. 
of  cardboard  in  between  them  and  four  straight  holes  bored  for  the  re- 
ception of  the  field-magnet  cores. 

Fig.  89  is  a  semi-sectional  elevation  of  the  field  magnet  complete 
without  the  journal  pedestals ;  a  field-magnet  core  is  shown  by  Fig.  90. 
The  cast-iron  pole-pieces  must  be  accurately  fitted  to  the  ends  of  the 
cores  and  pinned  permanently  in  place  with  iron  pins.     The  magnet  ring 
and  pole-pieces  should  be  of  the  best  grade  of  pig  iron  obtainable.     The 


ELECTRICAL  DESIGNS 


FIG.   89.— SEMI-SECTIONAL  ELEVATION  OF  THE  FIELD-MAGNET  FRAME. 

magnet  cores  should  be  made  of  Norway  wrought  iron.  The  corners  of 
the  pole-pieces  should  be  heavily  rounded  so  that  no  sharp  edges  are  left. 
The  length  of  the  pole-piece  parallel  with  the  shaft  is  the  same  as  its  width 
at  right  angles  to  this  dimension.  The  outside  diameter  of  the  magnet  ring 


THREE  HORSE-POU'ER  MOTOR 


81 


FOOT 


FIG0   90. — MAGNET  CORE. 


FIG.    QI.— SIDE  ELEVATION  OF  MAGNET  FRAME  AND  SECTION  THROUGH  BEARING. 

is  2oys  ins.     The  extreme  breadth  of  the  ring  parallel  with  the  shaft  is  8 

ins.     The  other  dimensions  are  as  below  : 

INCHES. 

A — Bore  of  armature  chamber 8 

B — Axial  length  of  pole-face 3& 

C— Width  of  pole-piece,  tip  to  tip,  in  a  straight  line 3^ 


82 


ELECTRICAL  DESIGNS 


D — Radius  to  which  pedestal  seat  is  cut , 8}£ 

E — Thickness  of  yoke  ring i/^ 

F — Distance  between  ribs 7 

G — Width  of  plane  surface  back  of  magnet  coil 4/4 

H — Height,  base  line  to  armature  center 12 

O — Axial  length  of  pedestal  seat 4 

P — Straight-line  width  of  pedestal  (width  of  pedestal  scat  is  the  same). .  7 

Q — Length  of  pedestal  foot,  commutator  side 7 

The  dimensions  of  the  magnet  core  (Fig.  90)  are  as  follows : 

At    y          m  p 

Diameter,  ins 2^4,         3^         lYz 

Lengthens 2  3  I1X 

Fig.  91  is  an  edge  view  of  the  field-magnet  frame,  including  one  jour- 
nal pedestal  shown  in  perspective  and  the  other  in  cross-section.  After 
fitting  the  magnet  cores  into  place  in  the  ring,  the  pole-pieces  should  be 
bored  and  the  pedestal  seats  cut,  at  one  setting  of  the  frame.  Fig.  92  is 

a  croSvS-section  of  the  armature 
core,  showing  the  details  of  con- 
struction. The  discs  are  mounted 
on  a  cast-iron  drum,  which  is 
provided  with  a  flange  head  and 
a  hub,  y,  at  one  end,  the  other 
end  being  open.  The  discs  are 
clamped  in  place  by  a  cast-iron 
ring  which  is  provided  with  a 
hub,  /,  similar  to  that  at  the  other 
end  of  the  drum,  and  drawn  to 
place  by  means  of  six  /^-in. 
bolts  passing  through  holes  in 
the  clamping  ring  and  tapping 
into  the  end  wall  of  the  cast-iron  drum.  The  center  lines  of  two  of  these 
bolts  are  indicated  by  b  b.  The  cast-iron  drum  should  have  a  key-seat  cut 
in  it  so  that  the  discs  may  be  positively  driven. 

Both  the  hub  on  the  end  of  the  drum  and  that  on  the  clamping  ring 
should  be  keyed  to  the  shaft  so  that  there  will  be  no  opportunity  for  dis- 
placement. At  each  end  of  the  core  structure  a  disc  of  fiber  1-16  in.  thick 
should  be  provided,  as  indicated  by  the  heavy  black  lines  in  the  engraving. 
These  discs  must  be  toothed  exactly  like  the  core  discs  so  that  the  ends  of 
the  magnetic  core  will  be  entirely  covered.  The  iron  core  discs  are  7^4 
ins.  in  diameter,  with  a  central  hole  4^/4  ins.  in  diameter,  and  47  slots  j4  m- 
wide  and  ^4  m-  deep ;  the  slots  have  parallel  sides.  The  discs  must  be  of 


FIG.    92. — SECTION  THROUGH  ARMATURE  CORE. 


THREE  HORSE-POWER  MOTOR  83 

the  best  grade  of  charcoal  iron,  1-40  in.  thick.    The  dimensions  of  the 
armature  core  structure  are  as  follows : 

INCHES. 
I — Length  of  hub  clear  through 2% 

Bore  of  this  hub iX 

J — Length  of  hub  clear  through 2 

Dore  of  this  hub I 

K — Diameter  of  hubs 2^ 

L — Internal  diameter  of  core  drum 4 

M — Outer  diameter  of  core  drum 4^ 

N — Diameter  of  flange  and  clamping  ring 6^ 

Thickness  of  flange  and  clamping  ring fi 

The  two  journal  pedestals  are  exactly  alike  and  made  of  ordinary 
cast-iron.  The  base  must  be  turned  accurately  to  conform  to  the  circle 
to  which  the  pedestal  seat  on  the  magnet  frame  is  machined.  The 
standard,  or  pedestal,  is  an  open  frame  of  j/2-in.  metal;  the  box  is  of  the 
ring-oiling  type,  with  a  single  ring  hung  exactly  midway  of  the  journal. 
Fig.  93  is  a  transverse  cross-section  of  the  pedestal  and  box.  The  box  is 
bushed ;  the  bushing  may  consist  of  a  brass  casting  turned  to  shape,  or  it 
may  be  made  by  babbitting  a  piece  of  thin  brass  tubing,  the  outer  di- 
ameter of  which  is  a  snug  fit  in  the  box.  The  oil  slot  across  the  center  of 

the  box  must  be  provided  with  a  suitable 
covering  to  exclude  dust.  Each  pedestal 
should  be  bolted  to  the  magnet  frame  with 
two  y% -in.  cap  screws.  The  pedestal  and  box 
measurements  are  below  : 

INCHES. 

P— Widest  part  of  standard 7 

R — Axial  length  of  oil  well 2^ 

S — Inner  length  of  oil  well 2 

T — Inner  width  of  oil  well 3 

t — Radius  line  to  indicate  origin  of  circle  ij£ 

U — Outside  diameter  of  box 2% 

V — Inside  diameter  of  box I  j£ 

W — Outside  length  of  box 4 

Y — Length  of  bushing 3^ 

Bore  of  bushing I 

Z — Projection  of  box  beyond  oil  well  wall.     -Jf 
fc I~?"-'l."pf'l"-~^ >|  Diameter  of  oil  ring 2^ 

FIG.  93.—  CROSS-SECTION    OF  Bore  °f  oil  rinS ^ 

PEDESTAL  AND  BOX.  Width  °f  °'d  rinS ^ 

A  bearing  must  be  turned  on  the  outside  of  the  inner  end  of  the 
pedestal  on  the  commutator  side  of  the  machine,,  as  indicated  in  Fig.  91,  to 


84  ELECTRICAL  DESIGNS 

accommodate  the  brush-holder  yoke,  which  may  be  copied  from  any  of 
the  standard  makes.  Only  two  sets  of  brushes  are  required,  each  set 
comprising  two  carbon  brushes  y$  in,  thick  and  ij/s  ins.  wide;  the  two 
sets  must  touch  the  commutator  exactly  90°  (n^4  segments)  apart,  center 
to  center.  The  commutator  must  have  47  segments,  and  must  measure 
3  ins.  along  the  shaft,  extreme  length.  The  commutator  core  must  be 
bored  to  fit  the  portion,  c,  of  the  shaft,  and  key-seated  to  correspond. 
The  diameter  of  the  barrel  should  be  not  less  than  4  ins.,  and  the  diameter 
measured  at  the  connecting  lugs  must  not  exceed  63/2  ins.  The  brush 
surface,  measured  parallel  with  the  shaft,  must  be  2.^/2  ins.  long.  It  will 
be  best  to  buy  the  commutator  complete  from  one  of  the  several  makers 
cf  this  class  cf  apparatus. 

Fig.  94  is  the  armature  shaft.     The  key-seats  are  all  J-fJ  wide  and  3-16 
deep.     The  dimensions  of  the  shaft  are  below: 


, At >          Total 

f  a  c  j  Length 

Diameter,  inches I  ilX-  1%         I 

Length,  inches 6-^        6j\         5-^        3^         21^ 

The  field-magnet  coils  may  be  wound  directly  on  the  cores  or  on 
bobbins  made  of  thin  vulcanized  fibre.  If  they  are  wound  directly  on  the 
cores,  the  latter  must  be  wrapped  first  with  three  layers  of  unbleached 
cottons  and  painted  with  shellac  varnish,  two  circular  coil  heads  of  hard 
fibre  being  first  fitted  to  the  large  part  of  each  core.  The  coils  consist  of 
No.  22  single  cotton-covered  wire,  wound  to  a  depth  of  ^4  in.,  exactly. 
The  exact  number  of  turns  is  immaterial,  except  that  all  four  coils  must 
contain  the  same  number  of  turns,  and  as  many  turns  should  be  put  on 
as  can  be  got  in  the  space  available.  With  careful  winding,  the  builder 
should  get  2,565  turns  in  each  coil.  For  a  23<>volt  motor  use  No.  25 
double  cotton-covered  wire,  and  for  500  volts  use  No.  28  double  cotton- 
covered,  wound  to  the  depth  specified.  Should  the  reader  prefer  to  wind 
the  coils  in  bobbins,  the  magnet  core  need  not  be  wrapped,  of  course. 
After  each  coil  is  completed,  secure  the  outer  end  and  cover  the  outside 
layer  with  unbleached  cottons  two  layers  deep,  heavily  varnished.  Fig. 
95  indicates  how  the  field  coils  should  be  connected  up. 

The  armature  coils  of  the  ii5~volt  machine  are  of  No.  13  wire,  each 
coil  containing  eight  turns.  The  winding  must  be  two  wires  wide  and 
four  layers  deep,  per  coil,  so  that  when  the  coils  are  in  place  there  will  be 
1 6  wires  in  each  slot — two  wide  and  eight  deep,  as  shown  in  Fig.  96. 
There  are  47  coils,  connected  up  wave-fashion.  In  winding  the  coils  it 
will  be  advisable  to  bend  a  hook  in  each  starting  end  and  leave  the  final 
ends  straight.  The  armature  coils  must  be  wound  in  a  former  so  that  the 


THREE  I!ORSE-PO::'ER  MOTOR 


FIG.   95. — FIELD  COIL  CONNECTIONS. 


outline  of  the  opening 
through  each  coil  is  a 
square  measuring  4^ 
ins.  on  each  side.  The 
ends  should  lead  out  from 
two  corners,  and  each 
coil  must  be  wrapped 
carefully  and  firmly  with 
two  layers  of  German 
linen  tape,  each  layer 
being  painted  with  shel- 
lac varnish.  The  slots  in 
the  armature  core  should 
be  provided  with  insulat- 
ing troughs  of  press 
board  1-64  in.  thick. 

Put  the  coils  on  the  armature  all  the  same  way — bent 
ends  to  the  left  and  straight  ends  to  the  right,  facing  the 
commutator.  There  must  be  twelve  teeth  between  the  two 
slots  in  which  any  given  coil  is  placed,  and  there  must  be  22 
commutator  segments  between  the  two  to  which  the  terminals 
of  any  given  coil  are  connected,  as  indicated  by  Fig.  97.  It 
will  be  found  best  to  first  put  on  1 2  coils  in  regular  right- 
handed  rotation,  pressing  the  ends  down  closely  where  they 

lap,  and  slipping  a  bit  of 
thin  oiled  paper  between 
the  crossings.  This  will 
put  one  layer  of  coils  in 
24  of  the  slots.  Then 
put  coils  in  the  23  vacant 
slots  in  the  same  fashion; 
there  will  then  be  46  half 
filled  slots  and  one  filled. 
Continue  the  second 
layer  of  coils  right  along 
from  the  24th  coil, 
following  the  same  plan 
as  before.  At  the  finish, 
there  will  be  a  bent  end 
and  a  straight  end  pro- 
PIG.  96.— SLOT.  FIG.  97.— CONNECTING  DIAGRAM.  jecting  from  each  slot. 


86  ELECTRICAL  DESIGNS 

Carry  the  bent  ends  1 1  or  1 2  segments  to  the  left,  around  the  commutator, 
and  put  them  all  in  the  segment  slots.  Then  take  any  one  of  the  straight 
ends,  find  the  bent  end  which  is  the  other  terminal  of  its  coil,  and  con- 
nect the  straight  end,  as  shown  in  Fig.  97,  with  22  segments  between  it 
and  its  mate.  The  other  straight  ends  may  be  put  in  in  regular  order 
without  tracing,  if  the  coils  have  been  put  on  the  core  properly  and  the 
bent  ends  in  the  commutator  lugs  in  strict  sequence. 

If  it  is  desired  to  build  the  machine  for  230  volts,  wind  the  armature 
with  No.  1 6  double  cotton-covered  wire,  putting  15  turns  in  each  coil — 
three  wide  and  five  deep — so  that  each  slot  will  contain  30  wires,  3  wide 
and  10  deep.  For  500  volts,  use  No.  19  wire,  putting  28  turns  in  each 
coil — 4  wide  and  7  deep — so  that  each  slot  will  contain  56  wires,  4  wide 
and  14  deep.  The  principal  technical  data  for  the  Ii5-volt  machine  are 
given  below : 

Revolutions  per  minute 1,320 

Armature  resistance,  warm 0.4 

Armature  current,  normal 22 

Armature  and  brush  drop,  volts  about 10 

Per  cent  regulation  about 9% 

Flux  density  in  armature  core  and  teeth,  per  square  inch.. .  75,000 

Flux  density  in  air  gap 29,300 

Flux  density  in  magnet  cores 93,000 

Flux  density  in  magnet  yoke : 48,000 

Leakage  coefficient 1.25 

Resistance  of  field  winding,  ohms 169 

Exciting  current,  amperes 0.68 

Copper  loss  in  field,  watts 78.2 

Copper  loss  in  armature,  watts IQ4 

Core  loss  in  armature,  watts 46 

Approximate  efficiency,  allowing  5  per  cent  for  friction  and 

windage,  per  cent 82 

The  starting  box  should  be  purchased  from  any  of  the  standard 
rheostat  builders ;  a  satisfactory  home-made  one  of  this  size  is  rarely 
produced. 


CHAPTER  X. 


ONK   KILOWATT   COMBINED  ALTERNATING  AND   DIRECT-CURRENT 

MACHINE. 


There  are  presented  in  this  chapter  designs  and  working  drawings 
for  a  type  of  combined  alternating  and  current  machine  which  it  is 
thought  will  prove  generally  useful  for  experimental  and  laboratory  work 
in  alternating  and  direct  currents,  and  which  is  applicable  on  most  of  the 
electric-lighting  circuits  found  in  practice. 

The  design  contemplates  working  the  machine  in  a  number  of  dif- 
ferent ways : 

1.  As  a  direct-current  generator  or  motor. 

2.  As  a  single,  two  or  three-phase  generator  or  motor. 

3.  As  a  rotary  converter,  changing  single,  two  or  three-phase  to 
direct  current. 

4.  As   an  inverted  rotary  converter,   changing  direct  current  to 
single,  two  or  three-phase  alternating  currents. 

5.  As  a  phase  transformer,  changing  alternating-  current  of  one 
phase  to  that  of  any  other  phase. 

Some  of  the  foregoing  functions  may  be  in  operation  at  the  same 
time ;  for  instance,  Nos.  i  and  2  combined  would  give  a  "double- 
current"  generator.  Also  No.  3  or  No.  4  may  be  in  operation  simultane- 
ously with  No.  5. 

Three  sizes  of  this  type  of  machine  will  be  described,  of  I,  2  and  4 
kilowatts  capacity,  respectively,  and  in  all  of  these  the  same  scale  of  volt- 
age has  been  adopted,  namely,  no  volts  for  the  direct  current,  80  volts 
for  single  or  two-phase  alternating,  and  70  volts  for  the  three-phase  alter- 
nating. These  voltages  admit  of  considerable  adjustment,  however,  by 
varying  the  field  excitation  or  speed  in  case  of  a  generator.  The  values 
given  represent  about  the  maximum  which  can  be  developed  continu- 
ously. 

In  operating  on  single-phase  alternating  circuits  it  is  necessary  to 
adopt  some  device  which  will  make  the  machine  self-starting,  and  this 


88  ELECTRICAL  DESIGNS 

has  been  provided  in  the  shape  of  a  special  switch  located  in  the  base 
of  the  machine  and  which,  at  starting,  temporarily  changes  the  connec- 
tions to  those  of  a  series  motor  which,  as  is  well  known,  readily  starts 
•when  alternating  current  is  turned  on.  The  armature  is  allowed  to  reach 
a  speed  slightly  above  synchronism,  and  the  switch  is  then  thrown  over 
to  the  running  position,  where  the  machine  operates  as  an  ordinary 
synchronous  motor. 

In  starting  on  two  or  three-phase  circuits,  the  same  switch  is  utilized 
to  break  up  the  field  winding  into  a  number  of  short  sections  on  open 
circuit,  thereby  avoiding  the  high  induced  e.m.fs.  which  would  otherwise 
be  produced  on  turning  the  alternating  current  into  the  armature  wind- 
ing. It  will  be  understood  that  where  two  or  three-phase  currents  are 
employed  the  machine  is  self-starting  without  any  special  device,  by 
virtue  of  the  rotary  field  principle.  If  the  starting  current,  with  this 
arrangement,  is  found  to  be  objectionably  large,  it  can  be  avoided  by 
starting  on  a  reduced  pressure  supplied  from  small  auto-transformers. 

The  general  features  of  the  design  are  multipolar  field  having  a 
circular  yoke  of  cast  iron  with  laminated  wrought-iron  poles  cast  in. 
This  type  is  selected  because  it  admits  of  high  magnetic  density  and  short 
air  gap.  and  consequently  much  greater  output  than  does  an  all  cast  field, 
while  at  the  same  .time  it  is  only  slightly  more  expensive  or  difficult  to 
construct.  An  all  cast-iron  field  of  the  same  general  design  will  have 
only  a  little  more  than  half  the  output,  and  an  all  cast-steel  field,  while 
good  magnetically,  is  scarcely  to  be  considered  at  present  owing  to  the 
difficulty  in  securing  steel  castings  on  short  notice. 

Field  coils  wound  in  two  or  more  sections  each,  and  provided  with 
terminals  for  connection  to  the  starting  switch.  This  is  necessary  in 
order  to  obtain  a  sufficient  reduction  in  the  impedance  by  connecting  the 
various  sections  in  multiple  at  the  start. 

A  distributed  armature  winding,  with  collector  rings  tapped  in  at 
appropriate  intervals  on  the  commutator  for  alternate-current  working. 
A  toothed  armature  core  with  deep  and  narrow  slots,  and  provided  with 
a  formed-coil  winding,  as  in  direct-current  practice. 

The  minimum  number  of  slots  and  coils  is  determined  by  the  number 
of  poles  and  by  the  consideration  that  taps  must  be  made  for  both  two*, 
and  three-phase  working.  The  quotient  obtained  by  dividing  the  number 
of  coils  or  commutator  segments  by  the  number  of  poles  must  be  divisible 
by  two  for  two-phase  working  and  by  three  for  three-phase  working,  and 
hence  by  two  times  three  for  both  together.  Thus  24  coils  and  segments 
are  appropriate  for  a  four-pole  machine,  36  for  a  six-pole,  and  so  on. 

Six  collector  rings  will   be   required;   ordinarily  seven  would  be 


ONE  KILOWATT  DOUBLE-CURRENT  MACHINE  89 

necessary,  four  for  two-phase  and  three  for  the  three-phase.  By  making 
one  of  the  two-phase  rings  the  starting  point  for  the  three-phase,  one  ring 
serves  for  two,  and  the  total  number  may  be  reduced  to  six. 

It  would  be  possible,  of  course,  to  use  but  four  rings,  obtaining 
three-  phase  current  by  means  of  two-phase  three-phase  transformers,  but 
it  is  preferable  to  add  two  rings  and  obtain  all  phases  directly-  from  the 
machine. 

The  hollow  base  plate,  which  is  cast  in  one  piece  with  the  bearing 
pedestals,  serves  as  a  housing  for  the  starting  switch  already  referred  to. 
This  switch  is  operated  by  a  lever  on  the  outside,  at  the  front  or  direct- 
current  end  of  the  machine,  and  has  two  positions  120  degrees  apart,  the 
starting  and  running  positions  respectively.  In  the  starting  position  the 
various  sections  of  the  field  winding  are  in  parallel  with  each  other  and 
in  series  with  the  direct-current  end  of  the  armature. 

In  the  running  position  the  field  sections  are  in  series,  giving  the 
maximum  resistance,  and  are  placed  across  the  direct-current  brushes, 
at  the  same  time  alternating  current  from  the  single-phase  mains  is 
turned  into  the  collector  rings. 

A  pulley  having  a  heavy  rim  for  the  purpose  of  securing  a  consider- 
able fly-wheel  effect  will  be  found  advantageous  in  adding  to  the  smooth 
running  of  the  machine,  particularly  when  used  as  a  rotary  from  the 
alternating-current  end. 

A  pulley  of  this  kind  will  also  be  useful  where  the  machine  is  to  be 
used  as  a  generator  direct  belted  to  a  gas  or  gasoline  engine.  The  need 
for  a  considerable  amount  of  momentum  in  the  running  parts  of  a  rotary 
is  real  and  genuine,  for  without  it  there  is  a  disagreeable  oscillation  or 
"pumping,"  which  makes  synchronism  unstable  and  sometimes  causes  the 
machine  to  break  out  of  step  even  before  full  load  is  reached. 

The  bearings  are  of  the  ring-oiling  type,  and  of  a  form  which  gives 
good  lubrication  without  the  disadvantage  of  having  oil  thrown  oil 
outside  the  bearing. 

The  running  qualities  of  these  machines  will  doubtless  prove  quite 
satisfactory.  There  is  not  likely  to  be  trouble  from  sparking,  in  spite  of 
the  fact  that  the  armature  is  multiple  wound,  in  which' ordinarily,  a  slight 
lack  of  symmetry  in  field  strength  would  cause  heating  and  sparking. 
The  connections  already  made  to  the  collector  rings  for  another  purpose 
serve  also  as  equalizers,  which  permit  equalizing  currents  to  flow  and 
thus  counteract  any  slight  inequality  in  the  various  field  poles. 

Armature  reaction  may  be  guarded  against  by  clipping  off  the 
corners  of  every  third  lamination  in  the  field  poles.  This  will  have  the 
effect  of  increasing  the  density  in  the  pole  tips  to  practical  saturation,  thus 


90  ELECTRICAL  DESIGNS 

avoiding  further  distortion  by  armature  currents  and  giving  practically  a 
fixed  point  of  commutation  for  all  loads. 

Heating  in  the  armature  and  field  windings  should  not  prove  serious, 
for  the  current  densities  employed  are  moderate,  considering  the  size  of 
machine.  In  the  pole  pieces,  heating  would  ordinarily  be  expected,  due 
to  the  short  air  gap  and  high  density,  but  their  laminated  construction  will 
entirely  obviate  this  difficulty. 

While  primarily  intended  for  use  on  125-cycle  circuits,  modifications 
will  be  indicated  enabling  these  machines  to  be  used  on  6o-cycle  circuits 
also.  This  involves  either  a  reduction  in  speed  of  one-half,  with  a 
correspondingly  reduced  output  and  voltage,  or  a  reduction  in  the  number 
of  poles  to  one-half,  keeping  the  speed  and  output  the  same,  but  necessi- 
tating a  somewhat  more  difficult  change  in  connections  and  winding. 

Referring  now  to  the  one-kilowatt  machine,  Fig.  98  shows  an  end 
yiew  of  the  field  magnet  and  base.  There  are  four  poles  cast  into  the 
yoke,  which  forms  a  separate  casting  and  is  bolted  to  the  base  plate  by 
four  7-i6-in.  by  il/2 -in.  hexagon  cap  screws.  The  poles  are  built  up  of 
plain  rectangular  strips  of  soft  iron  about  No.  22  gauge,  which  are 
clamped  between  two  heavier  plates  by  one  or  more  long  flat-head  bolts. 

The  pattern  for  the  field  casting  should  be  made  just  as  though  it 
were  for  an  all  cast  field,  the  laminated  pole  pieces  being  laid  in  the 
mould  after  the  pattern  has  been  drawn,  and  the  iron  poured  in  around 
them.  The  natural  shrinkage  of  the  metal  on  cooling  will  cause  the  poles 
to  be  tight  and  secure.  It  would  give  additional  security,  however,  to 
notch  the  poles  before  casting  in  as  indicated  by  the  dotted  lines.  Still 
another  plan  is  to  leave  the  end  plates  short,  and  to  spread  the  laminations 
apart  where  they  enter  the  yoke.  This  will  allow  the  iron  to  fill  in  the 
interstices  and  so  obtain  a  good  hold  on  the  pole.  As  the  poles  have 
teen  left  with  square  ends,  they  must  now  be  bored  out  3  5-16  ins.  and 
the  corners  slightly  rounded. 

Fig.  99  is  a  longitudinal  half  section  of  the  assembled  machine,  which 
shows  the  construction  of  the  armature,  bearings,  commutator,  and  col- 
lector rings,  and  also  the  location  of  the  starting  switch  in  the  base. 

The  armature  core  is  built  up  of  soft-iron  discs  about  No.  27  gauge ; 
two  heavier  discs  of  wrought  iron,  3-16  in.  thick,  are  provided  at  the  ends 
as  a  reinforcement  for  the  teeth,  and  the  whole  is  clamped  between  two 
cast-iron  flanges  run  up  on  threads  cut  in  the  shaft.  These  flanged  pieces 
serve  also  as  a  support  for  the  "straight-out"  winding. 

Plain  round  discs  may  be  used  in  building  the  core  and  the  slots 
milled  out,  being  careful,  however,  to  take  the  discs  apart  after  milling  and 
insulate  them  with  paper  or  japanning.  The  keyway  in  the  dies  insures 


ONE   KILOWATT   DOUBLE-CURRENT   MACHINE  91 

their  registering  when  reassembled,  in  spite  of  possible  slight  inaccuracy 
in  milling  the  slots.  It  is  not  necessary  to  insulate  the  discs  from  the  shaft 
if  they  are  fairly  well  insulated  at  all  other  points. 

The  bearings  have  a  central  rib  3/£  in.  thick,  which  supports  the 
brass  or  bronze  sleeve  forming  the  journal  proper.     The  oil  pockets  at 
1 


FIG.   98. — END  ELEVATION  OF  THE  MACHINE  WITHOUT  THE  ARMATURE. 

either  side  of  the  web  communicate  by  means  of  a  slot  cored  out  in  the 
web,  so  that  the  oil  level  may  remain  the  same  on  each  side.  The  oil 
rings  are  ^  in.  wide  and  ride  on  the  shaft  through  grooves  turned  eccen- 
trically in  the  sleeve. 


ONE  KILOWATT  DOUBLE-CURRENT  MACHINE 


93 


The  commutator  has  a  steel  sleeve  fitting  the  shaft,  upon  which  are 
two  flanges,  one  solid  with  the  sleeve  and  the  other  threaded  on  it  and 
tightened  by  means  of  a  spanner  wrench  applied  to  holes  drilled  in  its 
face.  Both  flanges  are  undercut  at  an  angle  of  about  60  degrees,  to  hold 
the  segments  in  place. 

Probably  the  best  way  to  construct  the  commutator  is  to  turn  up  a 
copper  casting  of  the  required  section,  and  then  slit  the  cylinder  into  24 
regments  by  means  of  a  i-32-in.  cutter,  in  a  milling  machine.  The  seg- 
ments are  then  built  up  with  i-32-in.  mica  between  and  insulated  from 
the  sleeve  by  1-16  in.  of  mica  or  other  good  insulation. 

The  collector  rings  are  similar  in  construction.  The  two  end  rings 
are  counter-bored  to  let  in  the  flanges  of  the  sleeve,  which,  in  this  case, 
need  not  be  undercut.  The  other  rings  are  plain  round  and  are  simply 
clipped  over  the  insulating  sleeve,  and  separated  from  each  other  by 
i-i6-in.  fiber,  or  equivalent  insulation,  which  is  allowed  to  project  some- 
what above  the  surface  of  the  rings. 

Connections  to  the  rings  arc  made  by  drilling  in  from  the  back  side 
and  soldering  in  short  wires,  No.  12  or  No.  14,  which  should  be  carefully 


I-'IGS.    100  AND   101. — BRUSH-HOLDER   COLLARS. 


FIG.  102. — ARMATURE  HEAD. 


insulated  where  they  pass  through  other  rings  by  small  fiber  or  rubber 
tubes.  These  wire  leads  are  made  only  just  long  enough  to  project  a 
short  distance  from  the  back  ring  and  are  there  soldered  to  some  thin 
copper  strips  taped  and  laid  in  the  bottom  of  the  armature  slots,  six  of 
which  have  been  cut  1-16  in.  deeper  than  the  rest  to  accommodate  these 
connections.  It  will  be  the  more  convenient  to  make  all  these  connec- 
tions permanently  and  test  them  before  laying  on  the  armature  coils. 

Fig.  100  shows  the  brush  ring  for  the  alternating  current  end,  and 
Fig.  101  the  one  for  the  direct-current  end  of  the  machine.  They  are 
made  in  halves,  held  together  by  screws,  which  will  facilitate  in  assemb- 
ling the  machine.  The  direct-current  ring  has  four  lugs  for  supporting 
the  brush  holder  and  the  alternating-current  ring  has  six,  one  for  each  of 
the  six  collector  rings. 


94 


ELECTRICAL  DESIGNS 


Fig.  102  shows  an  end  view  of  the  armature  core  and  Fig.  103  a 
development  of  the  armature  winding.  The  core  has  24  slots  3-16  in. 
wide  and  7-16  in.  deep.  Every  fourth  slot  is  made  y2  in.  deep  to  allow 
space  for  connections  to  the  rings.  The  teeth  are  plain  straight  and  the 
armature  must  be  banded  after  the  coils  are  in  place. 

The  armature  winding  is  of  the  type  known  as  "straight  out"  and  is 
composed  of  form-wound  coils  of  No.  20  double  cotton-covered  wire,  each 
coil  consisting  of  16  turns  arranged  four  wide  and  four  deep.  The  coil 
is  wound  as  a  simple  straight  loop,  and  after  receiving  a  wrapping  of  tape 
it  is  bent  until  it  will  span  one-quarter  of  the  armature  circumference. 
One  side  of  a  coil  occupies  the  top  of  a  slot  and  the  other  side  of  the 
same  coil  occupies  the  bottom  half  of  a  slot  90  degrees,  or  six  slots,  in 
advance  of  the  first.  Thus  arranged,  the  coils  interleave  in  a  very 
compact  manner  and  the  space  required  for  cross  connection  is  reduced  to 
a  minimum. 

The  terminals  are  brought  out  at  the  apex  of  the  coil  and  are  con- 
nected directly  to  the  commutator  segments,  the  beginning  of  one  coil 


T- 

.  -}  -  -  s'- 


FIG.   103. — BARREL  WINDING. 


---*• — H          r>»-' 

FIG.    104. — BRUSH-HOLDER  DETAILS. 


and  the  ending  of  the  adjacent  coil  connecting  to  the  same  segment 
The  advantage  in  bringing  the  terminals  straight  out  to  the  commutator 
in  this  way  is  that,  in  addition  to  being  more  convenient,  it  permits  the 
brushes  to  be  placed  opposite  the  poles,  where  they  are  more  accessible 
than  when  placed  between  the  poles. 

Fig.  104  shows  details  of  the  brush  holders.  The  direct-current 
holders  are  of  simple  construction,  but  neat  in  appearance,  and  are 
intended  for  radial  graphite  or  carbon  brushes  y%  in.  thick,  \y±  ins.  wide 
and  I  in.  long.  The  necessary  tension  on  the  brush  is  supplied  by  an 
open-coil  spring  concealed  in  a  hollow  lug  cast  on  the  side  of  the  holder, 
and  acting  on  a  small  pressure  foot  shown  separately  in  the  drawings. 


ONE  KILOWATT  DOUBLE-CURRENT  MACHINE  95 

By  lifting  the  pressure  foot  by  means  of  the  eye  at  its  top  and  turning  it 
half  around,  a  brush  may  be  readily  removed  from  or  inserted  into  the 
holder. 

The  alternating-current  brush  holders  are  carried  upon  studs  sup- 
ported from  the  brush  ring,  and  have  slots  l£  in.  by  y§  in.  for  copper-leaf 
brushes.  There  need  not  be  any  spring  tension  provided,  as  the  natural 
spring  of  the  brush  will  be  sufficient  to  insure  good  contact.  Two  thumb 
screws  are  provided,  one  to  hold  the  brush  and  the  -other  to  clamp  the 
holder  upon  its  stud  in  the  desired  position.  The  studs  are  of  different 
lengths,  the  dimension -marked  X  having  the  values  3J/£  ins.,  2?^  ins.,  2J/& 
ins.,  i y%  ins.,  ij£  ins.  and  y%  in.  for  the  six  studs.  Quarter-inch  brass  rod 
may  be  used  to  make  these  from,  the  collars  being  soldered  or  threaded 
on  and  the  ends  threaded  for  a  hexagon  nut.  All  brush  holders  and  parts 
should  be  made  in  brass  or  bronze. 

Figs.  105  and  106  are  diagrams  to  be  followed  in  making  taps  to  the 
collector  rings.  The  4-pole  arrangement,  Fig.  105,  is  intended  for  oper- 
ating on  i25-cycle  circuits  and  the  two-pole,  Fig.  106,  for  60  cycles. 
These  connections  should  be  made  at  the  back  of  the  commutator  before 
it  is  placed  in  position  on  the  shaft.  In  the  four-pole  arrangement,  for 


FTG.    105.— FOUR-POLE  CONNECTIONS.      FIG.  Io6.— TWO-POLE  CONNECTIONS. 

instance,  segments  No.  i  and  No.  13  are  connected  together  and  to  a  lead 
marked  No.  i,  which  goes  to  collector  ring  No.  I,  and  similarly  for  the 
others.  Thus  connected,  single-phase  current  may  be  obtained  from 
rings  1-2  or  3-4.  Two-phase  current  from  1-2  and  3-4  and  three-phase 
current  from  1-5-6.  The  output  and  voltage  with  these  various  connec- 
tions are  as  follows :  Direct  current,  10  amperes  at  no  volts ;  single-phase 
alternating,  10  amperes  at  80  volts  ;  two-phase  alternating,  7  amperes  per 
phase  at  80  volts;  three-phase  alternating,  6  amperes  per  phase  at  70 
volts. 

Fig.  107  shows  a  form  of  fly-wheel  pulley  which  is  recommended  as 
Conducing  to  smooth  running,  for  reasons  already  referred  to.    This 


ELECTRICAL  DESIGNS 


pulley  is  of  cast-iron  and  should  be  turned  perfectly  true  all  over  and 
carefully  balanced,  as  should  also  the  armature.  These  rotating  parts  will 
be  required  to  run  at  3750  r.p.m.  on  125  cycles,  and  unless  precautions  are 
taken  the  vibration  will  be  excessive. 

Fig.  1 08  is  a  detail  of  the  armature  shaft.  This  is  designed  to  be 
turned  from  a  piece  of  24-in.  cold-rolled  steel,  and  for  this  reason  the 
customary  collar  at  one  end  has  been  omitted,  and  instead  threads  are 
cut  on  both  ends  for  receiving  the  end  plates  of  the  core.  This  does 
away  with  expensive  forgings  and  provides  a  shaft  requiVmg  only  a 


FIG.   I08. — FLY-WHEEL  PULLEY. 


FIG.    109  A.— DETAILS  OF  THE  STARTER  SWITCH. 

minimum  amount  of  turning.  Small 
grooves  are  provided  at  the  journals 
which  prevent  oil  from  creeping  along 
the  shaft  and  being  thrown  off  outside 
the  bearing.  There  are  two  keys  oil  the 
shaft,  one  for  the  core  punchings  and 
the  other  to  hold  on  the  pulley. 

Fig.  109  shows  the  arrangement  cf  the  switch  cylinder  and  contacts 
for  the  single-phase  starting  device.  There  are  30  contact  fingers,  each 
5-16  in.  wide,  fastened  to  a  strip  of  fiber  %  in.  thick,  which  in  turn  is 
screwed  to  the  under  side  of  the  cast-iron  base  of  the  machine.  Upon  a 
cylinder  of  hard  wood  or  fiber  1^4  ins.  in  diameter  are  arranged  two  rows 
of  brass  pieces,  sunk  in  grooves  cut  on  the  cylinder  and  upon  which  the 
stationary  contact  fingers  press. 

The  cylinder  may  be  rotated  through  an  angle  of  120  degrees  by 
means  of  a  handle  on  the  outside.  The  contacts  on  the  cylinder  are  120 
degrees  apart,  which  allows  sufficient  space  for  the  first  set  to  leave 


98  ELECTRICAL   DESIGNS 

contact  before  the  second  comes  into  contact,  this  being  essential  to  avoid 
short  circuit. 

Fig.  no  shows  a  diagram  of  connections  for  the  starting  switch,  by 
means  of  which  its  action  may  be  readily  traced  out.  Numbers  1-12 
represent  the  sectional  field  winding,  there  being  four  coils,  each  of  which 
is  wound  in  three  sections  of  approximately  equal  resistance.  There  are 
then  twelve  pairs  of  ends  which  lead  down  into  the  base  of  the  machine 
and  are  connected  to  the  stationary  contact  pieces,  which  are  represented 
by  the  upper  row  of  small  circles.  The  remaining  three  pairs  of  contacts 
connect  to  the  d.c.  brush  leads,  the  single-phase  rings  and  the  single- 
phase  mains  respectively. 

The  lower  rows  of  circles  represent  the  contact  pieces  mounted  on 
the  cylinder,  and  these  are  connected,  as  here  indicated,  by  means  of 
wires  laid  in  grooves  upon  the  cylinder  and  occupying  that  portion  of  the 
cylinder  over  which  the  contact  fingers  do  not  pass. 

To  operate  the  machine  at  no  volts  direct  current  or  I25~cycle 
alternating,  no  changes  are  necessary.  For  6o-cycle  alternating,  how- 
ever, the  number  of  poles  is  reduced  one-half  by  reversing  the  terminals 
of  any  two  successive  field  coils,  and  the  armature  winding  must  be 
changed  to  a  bipolar,  one. 

Another  plan  is  to  reduce  the  speed  one-half,  thus  halving  the  volt- 
age and  output  and  connecting  the  field  coils  in  series-multiple  so  that 
they  will  still  take  the  same  current  as  at  the  higher  voltage.  In  operat- 
ing the  machine  as  a  converter,  if  it  is  desired  that  the  direct-current  out- 
put be  at  no  volts, the  single  or  two-phase  input  must  be  at  80  volts.  This 
relation  of  voltage  is  fixed  and  can  be  expressed  by  d.  c.  volts  X  707 
=  a.c.  volts,  and  for  three-phase  by  d.  c.  volts  X  -612  =  a.c.  volts.  So 
that  if  the  alternating  circuit  is  of  52  or  104  volts  the  machine  should  be 
supplied  at  the  proper  voltage  through  a  transformer.  An  old  i5-light 
transformer  will  serve  for  this  purpose,  and  it  should  be  arranged  so  that 
its  secondary  voltage  can  be  varied  to  some  extent  by  changing  the 
number  of  secondary  turns  in  circuit,  thus  giving  a  means  of  adjusting 
the  direct-current  voltage. 

The  following  is  a  brief  summary  of  the  data  for  winding  and  general 
dimensions,  and  shows  the  method  of  calculating  same : 

Four-pole  machine,  3750  r.p.m. ;  armature,  3^  ins.  diameter,  3  ins. 
long;  24  slots,  3-16  in.  wide,  7-16  in.  deep;  total  number  of  conductors, 
768 ;  24  coils,  No.  20  wire,  4  wide,  4  deep ;  No.  20  has  1021  circ.  mils,  di- 
ameter d.c.c.,  .042  in. ;  direct-current  output  at  400  c.m.  per  ampere,  10 

amperes  ;  useful  lines  per  pole  -  ^        —  =  240,000 ;  total  lines,  330,000. 

700  /\  02.5 


ONE  KILOWATT  DOUBLE- CURRENT  MACHINE 


99 


Part 

Material 

Total  lines 

Cross  sect. 

B. 

H.. 

L. 

Amp.  turns 

Armature 

Wrought  iron 

120,000 

2.25  sq.  ins. 

53.300 

14 

1.4    in. 

20 

2  air  gaps 

Air 

240,000 

4-5 

53,300 

16,800 

.06  " 

1,000 

4  teeth 

Wrought  iron 

240,000 

2. 

120,000 

1  80 

•45  " 

So 

2  cores 

Wrought  iron 

330,000 

3-75      " 

88,oco 

20 

1-5     " 

30 

I  yoke 

Cast  iron 

165,000 

4- 

41,300 

74 

4-25  " 

315 

Total 1445 


The  table  above  gives  a  total  of  1445  ampere-turns  or  725  ampere- 
turns  per  coil  ;  mean  length,  I  turn,  II  inches. 

Circ.  mils  shunt  wire    IT  X  11  X  7^5    _ 

25  X   12 

Use  No.  25  wire,  320  c.m.,  .028  inch  d.c.c.  1155  turns  (approximate) 
per  coil;  25  layers,  45  turns  wide. 

Wind  in  three  sections.     Bring  out  terminals  from  each  section. 

4  X  ! 


Resistance  of  shunt  field  = 


X  '97  =  136  ohms. 


Normal  shunt  current,  .63  ampere.    Use  a  rheostat  of  about  50  ohms 
total  resistance  in  shunt-field  circuit. 

Weight  of  wire  in  shunt  coils=  4X  III5*TI  X'97  —  4-  1  pounds. 

1000  X  12 

Length  of  wire,  each  armature  coil  =  --  —  ---  =  17.4  feet. 
Total  length  of  wire,  armature,  =  24  X  17.4  =  4*7  feet- 


Total  weight  of  armature  wire  = 


=  I>3  Poun<*So 


'3  —  .26  ohm. 


Resistance  of  armature  —  v 

1000  X  io 

Drop  in  armature  at  full  load  =  10.63  X  .26  =  2.76  volts. 


CHAPTER  XL 


TWO   KILOWATT    COMBINED    ALTERNATING    AND   DIRECT- CURRENT 

MACHINE. 


The  2-kw.  machine  shown  in  the  accompanying  drawings  is  similar  in 
design,  construction  and  operation  to  the  four-pole  machine  described 
in  the  preceding  chapters.  The  present  machine  is  somewhat  larger, 
runs  at  a  slower  speed,  and  has  about  double  the  output  capacity  of  the 
four-pole  machine.  Fig.  in  gives  an  end  view  of  the  field-magnet 
frame.  There  are  six  poles  of  laminated  iron  cast  into  a  circular  yoke  of 
cast-iron,  which,  in  turn,  is  bolted  to  the  base  plate  by  four  hexagon  cap 
screws.  After  the  poles  are  cast  in  and  it  is  seen  that  all  of  them  are  tight 
and  firm  in  the  yoke,  they  may  be  bored  out  to  the  proper  diameter,  4.04 
ins.  The  armature  is  to  be  finished  4  ins.  in  diameter,  so  that  the  air-gap 
will  be  .02  in.  across  at  each  pole ;  this  will  be  ample  for  clearance  if  care  is 
taken  in  lining  up  the  machine. 

Fig.  112  is  a  section  of  the  assembled  machine  which  shows  the 
construction  and  relation  of  the  various  parts.  The  armature  is  of  the 
usual  laminated  construction,  the  core  discs  being  held  between  two  cast- 
iron  flanges  screwed  upon  the  shaft.  If  the  armature  slots  are  milled  out, 
the  discs  must  be  taken  apart,  cleaned  up,  and  insulated  before  being  fin- 
ally assembled  on  the  shaft.  If  this  is  not  done  the  eddy  current  loss  will 
be  excessive,  causing  heating  and  seriously  reducing  the  available  output. 
Fig.  113  shows  a  detail  of  the  armature  shaft.  This  is  intended  to  bo 
made  from  i-in.  cold-rolled  steel.  Threads  are  cut  at  both  ends  of  the 
core  portion  to  receive  the  cast-iron  flanges  which  clamp  the  core  punch- 
ings.  Two  keys  are  provided,  as  shown  in  the  drawing. 

The  bearings  are  made  with  a  brass  sleeve  fitting  the  shaft,  supported 
at  its  center  by  a  projecting  web  cast  in  the  bracket.  Although  it  is  pre- 
ferable to  bore  the  bracket  for  this  sleeve,  the  machine  work  may  be 
avoided  by  coring  the  bracket  somewhat  larger  and  then  babbiting  the 
sleeve  into  its  support  when  the  parts  have  been  lined  up  in  their  proper 
position.  The  oil  rings  are  of  brass  l/&  in.  wide  and  iJ/£  ins.  inside  diame- 


TWO   KILOWATT  DOUBLE-CURRENT  MACHINE 


101 


ter ;  grooves  are  cut  eccentrically  in  the  sleeves  to  receive  the  rings,  the 
grooves  being  made  about  5-32  in.  wide  in  order  to  allow  the  ring  a  small 
amount  of  play. 

The  commutator  is  built  up  on  a  machine  steel  sleeve,  with  the 
flanges  undercut  at  60  degrees.  The  segments  may  be  cast  separately  or 
cast  as  a  solid  cylinder  and  afterward  cut  into  segments  on  a  milling 


FIG.    III. — END  ELEVATION  OF  THE  FIELD-MAGNET  FRAME  AND  ONE  PEDESTAL. 

machine.  The  segments  should  be  of  copper,  with  i-32-in.  mica  between 
them,  and  i-i6-in.  micanite,  or  equivalent  insulation,  between  the  sleeve 
and  segments.  The  number  of  segments  is  36.  The  collector  rings  are 
six  in  number  and  mounted  upon  a  sleeve  with  3-32  in.  fiber  discs  between 
the  rings.  Connections  are  made  by  drilling  in  from  the  back  and  solder- 
ing in  short  leads  of  No.  8  or  No.  10  wire,  one  to  each  ring.  Each  of  the 


TWO   KILOWATT  DOUBLE-CURRENT  MACHINE 


leads  must  be  carefully  insulated  from  all  rings,  except  the  particular  one 
to  which  it  is  electrically  connected.  Some  thin  copper  strips  are  to  be 
provided  with  a  wrapping  of  tape  and  laid  in  the  bottom  of  the  armature 
slots,  six  of  which  must  be  made  1-16  in.  deeper  than  the  rest  to  accom- 
modate the  strips.  These  strips  carry  the  current  across  the  armature 
and  are  connected  to  the  commutator  at  the  proper  intervals. 

At  the  alternating-current  end  of  the  machine  the  fly-wheel  pulley  is 
shown  in  position  on  the  shaft.  This  style  of  pulley  will  be  found  advan- 
tageous in  operating  the  machine  as  a  rotary  converter  or  in  driving  it  by 
means  of  a  gas  engine.  If  the  machine  be  used  as  a  motor  an  ordinary 
pulley  will  answer.  The  pulley  is  for  a  2^-in.  belt,  and  is  $y2  ins.  in 
diameter. 

Fig.  114  shows  the  brush-holder  collar.  This  answers  for  both  the 
alternating-current  and  the  continuous-current  ends  of  the  machine,  as 


FIG.    114. — BRUSH-HOLDER  COLLAR. 


FIG.    115. — DETAILS   OF   BRUSH-HOLDERS. 


there  are  six  collector  rings  and  also  six  brush  holders.  Care  should  be 
taken  in  drilling  the  holes  for  brush  holders  to  have  them  equidistant,  for 
upon  this  the  accuracy  in  spacing  the  brushes  around  the  commutator 
depends.  At  the  alternating-current  end  this  does  not  matter  particu- 
larly. The  brush-holder  collars  are  necessarily  made  in  halves,  as  it 
would  be  difficult  to  assemble  the  machine  with  a  one-piece  collar. 

Fig.  115  shows  details  of  the  brush  holders.  These  are  of  the  same 
type  as  those  already  described  in  connection  with  the  four-pole  machine. 
The  alternating-current  brush  holders  have  no  spring  tension  and  are 
designed  for  leaf-copper  brushes  %  in.  thick  and  ^  in.  wide.  The  studs 
are  of  different  lengths  to  suit  the  position  of  the  rings ;  the  dimension, 
X,  is  32/s  ins.,  2%  ins.,  2*4  ins.,  ift  ins.,  I  3-16  ins.  and  11-16  inches  for 


104 


ELECTRICAL   DESIGNS 


the  six  studs.  They  are  made  of  5-16-111.  brass  rod.  The  continuous- 
current  brush  holders  are  designed  for  radial  carbon  brushes  j£  in.  thick, 
iJ4  ms-  wide  and  1%  ins.  long. 

Fig.  116  is  an  end  view  of  the  armature  core.  There  are  36  slots,, 
each  3-16  in.  wide  and  7-16  in.  deep;  every  sixth  slot  is  made  y2  in.  deep 
to  allow  space  for  the  connection  strips  referred  to  above.  After  the 
coils  are  in  place  the  armature  must  be  banded  at  three  points,  one  band 
to  go  around  the  center  of  the  core,  and  one  around  each  end  of  the 
winding  where  it  projects  beyond  the  core.  A  groove  must  be  turned  in 
the  periphery  of  the  core  to  accommodate  the  central  band,  so  that  the 
thickness  of  the  band  will  not  be  added  to  the  length  of  the  air-gap.  This 
groove  may  be  turned  on  the  core  before  the  slots  are  milled  out,  or  it 
may  be  done  afterward  by  filling  in  the  slots  temporarily  with  hard-wood 
strips.  It  should  be  about  1-16  in.  deep  and  ^g  or  7-16  in.  wide. 

Fig.  117  shows  a  development  of  the  armature  winding.  This  is  of 
the  "straight-out''  type,  and  is  composed  of  36  form-wound  coils  of  No.  20 
wire,  16  turns  per  coil.  One  side  of  a  coil  occupies  the  top  half  of  slot 
No.  i,  and  the  other  side  of  the  same  coil  occupies  the  bottom  of  slot  No. 
7;  that  is  to  say,  each  coil  spans  one-sixth  of  the  circumference  of  the 
core.  The  terminals  are  brought  out  at  the  apex  of  the  coil,  and  each  is 


FIG.    Il6. — END   OF   ARMATURE   CORE. 


FIG.    117. — DEVELOPMENT   OF  WINDING. 


connected  to  the  nearest  commutator  segment ;  the  inside  terminal  of  one 
coil  and  the  outside  terminal  of  the  adjacent  coil  connect  to  the  same 
segment.  The  point  of  commutation  will  be  found  at  or  near  the  center 
line  of  the  pole  pieces. 

Fig.  1 1 8  is  a  diagram  of  the  connections  for  the  collector  rings.  This, 
arrangement  is  for  a  six-pole  field.  The  leads  numbered  I  to  6  pass 
across  the  armature  and  are  connected  to  the  six  collector  rings  at  the 
alternating-current  end  of  the  machine.  Connected  in  this  way,  single- 
phase  current  may  be  obtained  from  rings  I  and  2  or  3  and  4,  two-phase 


TWO   KILOWATT  DOUBLE-CURRENT  MACHINE 


105 


currents  from  rings  I  and  2  and  3  and  4,  and  three-phase  currents  from 
rings  i,  $  and  6. 

The  output  and  voltage  with  each  of  these  various  methods  of  work- 
ing are  as  follows:  Direct  current,  15  amperes  at  115  volts;  single-phase 
alternating,  15  amperes  at  80  volts;  two-phase  alternating,  n  amperes  per 
phase  at  80  volts;  three-phase  alternating,  9  amperes  per  phase  at  70 
volts. 

Fig.  119  shows  the  outline  of  one  of  the  field  coils.  These  are  wound 
on  a  form,  and  each  coil  is  divided  into  two  sections  of  approximately 


FIG.   1 1 8. — DIAGRAM   OF  TAP  CONNECTIONS. 


FIG.    119. — FIELD   COIL. 


equal  resistance,  with  separate  terminals  brought  out  from  each  section. 
The  size  of  wire  is  No.  23,  B.  &  S.  gauge. 

The  arrangement  employed  for  starting  the  machine  as  a  motor  on 
single-phase  circuits  is  as  shown  in  the  description  of  the  four-pole  ma- 
chine (see  Fig.  109  and  no,  and  the  description  on  pages  96  and  97,  with 
the  single  exception  that  in  the  present  machine  there  are  six  coils  of  two- 
sections  each  instead  of  four  coils  of  three  sections  each. 

To  operate  the  machine  at  no  volts,  continuous  current,  or  125 
cycles  alternating,  the  speed  should  be  2500  r.p.m.  For  60  cycles  the 
only  method  available  is  to  reduce  the  speed  to  1200  r.p.m  and  to  connect 
the  field  winding  in  series  multiple.  This  is  most  conveniently  done  at 
the  starting  switch  by  changing  the  wiring  of  the  last  row  of  contacts  on 
the  switch  cylinder,  so  that  when  the  switch  is  in  the  running  position 
the  two  sections  of  each  field  coil  will  be  in  multiple  and  the  six  multi- 
plied pairs  in  series  and  connected  across  the  continuous-current  brushes. 
This  will  reduce  the  voltage  to  about  one-half  of  its  value  at  the  higher 
speed,  and  the  output  will  then  be  as  follows :  Continuous  current,  15 
amperes  at  55  volts;  single-phase,  15  amperes  at  40  volts;  two-phase,  II 
amperes  at  40  volts ;  three-phase,  9  amperes  at  35  volts.  It  is  probable 


io6 


ELECTRICAL   DESIGNS 


that  by  adjusting  the  field  excitation,  the  voltage  could  be  brought  up  to 
45  or  47  volts,  and  thus  admit  of  working  directly  on  single-phase  circuits 
of  50  or  52  volts  as  a  motor  or  rotary  without  the  use  of  an  individual 
transformer.  For  other  voltages  a  transformer  will  be  necessary. 

The  following  is  a  summary  of  the  data  for  winding  and  general 
dimensions :  Speed,  2500  r.p.m.  on  125  cycles,  or  1200  r.p.m.  on  60  cycles. 
Cast-iron  yoke,  laminated-iron  poles  cast  in.  Armature,  4  ins.  in  diame- 
ter, 3  ins.  long;  36  slots  3-16  in.  wide,  7-16  in.  deep;  every  sixth  slot  yl 
an.  deep,  36  coils  of  No.  20  wire;  16  turns  per  coil,  four  wires  wide  and 
Jour  deep.  Total,  1152  conductors.  At  15  amperes  continuous-current 
output  the  cross-section  of  armature  conductors  is  400  circ.  mils  per 
ampere. 

Useful  lines  per  pole,  at  115  volts  and  2500  r.p.m.  : 


1152  X  41-6 
US  X  10" 


—  240,000. 


TOTAL   LINES    PER   POLE,    32O,OOO 


Part 

Tptal  lines 

Cross  sect, 
sq.  in. 

B. 

H. 

Length 

Ampere 
turns 

120,000 

a. 

40,000 

IO 

1.5" 

je 

:2  air  gaps  

240  ooo 

4. 

60  ooo 

1  8  800 

O4 

•7SO 

240,000 

2. 

1  2O  OOO 

1  80 

44 

7Q 

160,000 

a.c 

46,000 

1  02 

•3. 

1  06 

^Z  cores   .    .......... 

320,000 

•I    -JC 

85  ooo 

18 

2t 

AS. 

23)  = 


Total  ampere  turns  in  field  winding,  995.    Circ.  mils  field  wire  (No. 
ii   X   12  X  500 


=   345.      Mean   length   per   turn,    12   inches. 


i6X  12 

Turns  (approx.)  per  coil,  500;  16  layers  of  32  turns  each.  Resis.  of  field 
winding  (coils  in  series),  63  ohms.  Normal  shunt  current,  I  ampere 
(nearly).  Use  rheostat  of  about  40  ohms  total  in  field  circuit. 

Mean  length  of  wire  per  armature  coil,  16  feet.  Total  length  of 
armature  wire,  36  X  16  =  610  feet.  Total  weight  of  armature  wire,  2 
pounds.  Resistance  of  armature,  0.17  ohm.  Drop  in  armature  winding 
at  full  load,  2^/2  volts. 


CHAPTER  XII. 


FOUR  KII<OWATT  COMBINED    ALTERNATING    AND    DIRECT-CURRENT? 

MACHINE. 


The  machine  here  illustrated  is  the  largest  of  the  machines  of  the 
same  general  type  of  which  this  is  the  third  to  be  described  in  this  book. 
The  present  machine  has  8  poles;  its  speed  is  from  1800  to  1875  r.p.m., 
and  it  has  an  output  capacity  of  four  kilowatts. 

Fig.  120  shows  an  end  view  of  the  field,  base,  and  bearing  pedestals. 
The  field  has  a  circular  yoke  of  cast-iron  with  pole-pieces  of  laminated 
wrought  iron  cast  in.  About  No.  20  gauge  iron  may  be  used  in  the  poles 
and  they  are  bored  out  to  5  9-16  ins.  diameter  after  being  cast  in.  Fig, 
121  is  a  section  of  the  assembled  machine,  which  shows  the  construction 
and  relation  of  the  various  parts.  The  armature  core  is  built  up  of  soft 
iron  discs  about  No.  27  gauge,  having  an  external  diameter  of  $J/2  ins.,, 
with  a  2^2-in.  hole  in  the  center.  The  discs  are  mounted  upon  three-arm 
spiders,  one  at  either  end  of  the  core,  and  the  arms  of  which  intermesh 
about  1/2  in.  at  the  center  of  the  core,' so  that  all  the  discs  are  supported  at 
least  three  points,  and  at  the  same  time  the  air  has  free  access  to  the 
interior  of  the  core.  Distance  pieces  are  provided  at  two  points  in  the 
core  which  divide  the  laminations  into  three  groups  with  3~i6-in.  ventilat- 
ing ducts  between  them.  Two  hexagonal  nuts  upon  the  shaft  provide 
means  for  clamping  the  core  discs  and  spiders. 

The  commutator,  which  is  shown  partly  in  section,  is  3^2  ins.  in 
diameter  and  i3^  ins.  wide  on  the  face.  There  are  48  segments  of  copper 
with  3j^-in.  mica  between  them,  and  3-32-in.  insulation  separates  the 
segments  from  their  supporting  sleeve.  The  sleeve  is  of  machine  steel 
and  has  flanges  undercut  at  an  angle  of  60  deg.  The  collector  rings  are 
six  in  number  and  are  made  of  copper.  The  rings  at  the  ends  are  J^-in, 
wide ;  the  rest  are  ^2  in. ;  j£-in.  insulation  separates  the  rings  from  each 
other.  The  bearings  have  a  brass  sleeve  fitting  the  shaft,  and  this  is  slot- 
ted to  allow  oil  rings  5-32  in.  wide  to  revolve  freely  with  the  shaft. 

Fig.  122  is  a  detail  of  the  armature  shaft.     This  is  designed  to  be 


ELECTRICAL  DESIGNS 


made  from  a  piece  of  i;4~in.  cold-rolled  steel.  Threads  are  cut  at  both 
ends  of  the  core  portion  to  receive  the  hexagonal  nuts  which  compress 
the  core  discs. 

At  the  alternating-current  end  of  the  machine  the  fly-wheel  pulley  is 
shown  in  position  upon  the  shaft.  A  pulley  of  this  kind  will  be  found  very 
useful  in  operating  the  machine  as  a  rotary  or  in  connection  with  a  gas 


FIG.    120. — END  ELEVATION  OF  FIELD-MAGNET  FRAME  AND  ONE  PEDESTAL. 

engine  for  driving,  as  it  assists  the  armature  in  maintaining  a  uniform 
rate  of  rotation. 

Figs.  123  and  124  show  the  brush-holder  collars  or  so-called  "quad- 
rants" for  the  alternating-current  and  direct-current  ends  of  the  machine. 
The  one  at  the  direct-current  end  has  eight  lugs  for  supporting  the  eight 
fcrush  holders  and  the  alternating-current  end  has  six,  one  for  each  of  the 


-jlr 


r  2 


no 


ELECTRICAL   DESIGNS 


—3 — - 


FIGS.    123  AND  124. — BRUSH  HOLDER  COLLARS. 


FIG.    125. — DETAILS  OF  BRUSH  HOLDERS. 


FOUR   KILOWATT  DOUBLE-CURRENT   MACHINE 


HI 


six  rings.     Both  are  made  in  halves  and  screwed  together  after  being 
placed  in  position  on  the  bearings. 

Fig.  125  shows  details  of  the  direct-current  and  alternating-current 
brush  holders.  The  direct-current  holders  are  for  radial  brushes,  ft  in. 
thick,  \y2  ins.  wide  and  ij4  ins.  long,  and  are  provided  with  a  spring 
tension  arrangement,  the  details  of  which  are  shown  in  the  engraving. 
The  alternating-current  brush  holders  are  for  copper-leaf  brushes  ft  in. 
thick  and  y2  in.  wide ;  the  spring  of  the  brush  itself  will  be  found  sufficient 
to  give  proper  contact  with  the  collector  rings.  The  studs  which  support 
these  holders  are  of  different  lengths  to  suit  the  position  of  the  various 
collector  rings.  The  dimension 
marked  x  on  the  drawings  has  the 
values  3^6  ins.,  3^  ins.,  2^  ins.,  2 
ins. ,  1 3/%  ins.  and  j£  in.  for  the  six 
studs  respectfully.  They  are  best 
made  of  4-in.  brass  rod. 


t—a-J 


FIG.    126. — END   OF  ARMATURE    CORE. 


FIGS.    127  AND  128. — DEVELOPMENT 
OF  WINDINGS. 


Fig.  126  shows  an  end  view  of  the  armature  core.  There  are  48 
slots,  3-16  in.  wide  and  J/2  in.  deep.  If  these  slots  are  milled  out,  it  will 
be  necessary  to  take  the  discs  apart  after  this  operation  in  order  to  anneal 
and  insulate  them  before  the  final  assembling.  Annealing  will  improve 
the  discs,  which  will  have  become  somewhat  hardened  from  the  machine 
work  which  has  been  done  upon  them. 

Figs.  127  and  128  show  developments  of  the  armature  winding. 
This  is  of  the  "straight-out"  type  and  is  of  two  forms,  known  as  the  "short 
coil"  (Fig.  127),  and  "long  coil"  (Fig.  128).  The  long  coil  is  for  use  in  a 
four-pole  field  and  for  6o-cycle  work.  The  short  coil  is  for  eight  poles 
and  125  cycles.  These  coils  are  form  wound  of  No.  17  double  cotton- 


112 


ELECTRICAL  DESIGNS 


FIG.    129. — EIGHT-POLE  TAP  CONNECTIONS. 

covered  wire,  12  turns  per  coil.  The  terminals  of  each 
coil  are  brought  out  at  its  apex,  and  are  connected  to 
the  two  nearest  commutator  segments,  inside  terminal 
of  one  coil  and  the  outside  terminal  of  the  adjacent 
coil  connecting  to  the  same  segment.  This  will  bring 
the  neutral  point  or  line  of  commutation  at  or  near  the 
center  of  the  pole-pieces. 


FIG.    130. — FOUR-POLE  TAP   CONNECTIONS. 


Figs.  129  and  130  are  diagrams  to  guide  in  making  taps  to  the  col- 
lector rings.  The  eight-pole  arrangement  is  intended  for  operating  on 
125-cycle  circuits,  and  the  four-pole  for  6o-cycle  circuit.  The  necessary 


FOUR  KILOWATT  DOUBLE-CURRENT  MACHINE 


interconnections  between  segments  may  be 
made  at  the  back  of  the  commutator  before  it 
is  placed  in  position  on  the  shaft.  For  instance, 
in  the  eight-pole  arrangement,  segments  i,  13, 
25  and  37  are  connected  together  and  to  a  lead 
marked  No.  i.  The  leads  numbered  i  to  6 
inclusive  pass  through  the  air  space  in  the 
center  of  the  core  and  connect  to  the  cor- 
responding rings.  Thus  connected,  single- 
phase  current  may  be  obtained  from  rings  i — 
2  or  3 — 4  ;  two-phase  currents  from  i — 2  and 
3 — 4,  and  three-phase  currents  from  i — 5 — 6. 
The  outputs  and  voltages  are  as  follows  : 

Amperes.  Voltage. 

Direct  current 35  115 

Single-phase  alternating.  .         35  80 

Two-phase  alternating. . .          25     per  phase      80 
Three-phase  alternating..         21        "       "          70 

Figs.  131  and  132  show  the  details  of  the 
switch  cylinder  and  contacts  which  are  located 


-120-^ 
FIG.    132. — DETAIL   OF  STARTING  SWITCH. 

in  the  base  of  the  machine  and  serve  as  a 
starting  device  for  operating  on  single-phase 
circuits. 

A  strip  of  fibre  19  ins.  long,  i  in.  wide  and 
|4  in.  thick  is  fastened  to  the  under  side  of 
the  cast  iron  base  and  upon  this  are  mounted 
38  contact  springs,  each  f/6  in.  wide.  The 
cylinder  is  of  hard  wood  i  y2  ins.  in  diameter, 
and  is  of  the  proper  length  to  just  go  inside 
the  base  and  have  its  ends  journaled  therein. 
The  free  ends  of  the  contact  springs  press  upon 
the  cylinder  and  make  connections  with  the 


1° 


to- 

o- 
o 

o- 


3 


•c 

*i  —  o 


3 
D 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 


H4  ELECTRICAL  DESIGNS 

contacts  fastened  upon  the  cylinder,  but  they  can  only  be  in  connection 
with  one  row  at  a  time.    Fig.  132  is  an  end  view  showing  details. 

The  diagram,  Fig.  133,  shows  how  the  connections  are  made  to  the 
switch  contacts.  The  uppermost  row  of  small  circles  indicates  the  con- 
tact springs  to  which  the  sectional  field  winding  is  connected.  There 
are  eight  coils  of  two  sections  each,  making  16  pairs  of  terminals  to  be 
connected  to  the  switch.  The  remaining  three  pairs  of  contacts  connect 
with  the  direct-current  brush  leads,  alternating-current  rings,  and  single- 
phase  mains  in  the  manner  indicated.  The  lower  two  rows  of  circles 
represent  the  contact  pieces  upon  the  revolving  switch  cylinder,  these 
being  simply  connected  in  groups  by  means  of  short  wires  or  metal  strips 
fastened  upon  the  cylinder  itself  and  occupying  that  position  of  the 
cylinder  over  which  the  contact  springs  are  not  required  to  pass. 

The  action  of  this  device  may  readily  be  followed  by  assuming  that 
the  "starting"  row  of  contacts  has  been  moved  up  to  engage  the  contact 
springs  when  it  will  be  seen  that  all  the  field  sections  are  in  multiple,  the 
armature  in  series  with  them,  and  the  whole  placed  across  the  single- 
phase  alternating-current  mains,  so  that  the  machine  starts  as  a  series 
motor.  When  the  second  row  of  contacts  engages  the  contact  springs 
the  field  sections  will  be  in  series  and  placed  in  shunt  across  the  direct- 
current  brushes,  while  at  the  same  time  the  single-phase  supply  current  is 
connected  to  the  collector  rings,  and  the  machine  is  now  operating  as  a 
synchronous  motor,  exciting  its  field  from  the  direct-current  end,  which 
is  the  normal  running  condition. 

To  operate  the  machine  at  no  volts  direct  current  or  125-cycle 
alternating-current  no  changes  are  necessary  and  the  speed  will  be  1875 
r.p.m.  For  6o-cycle  alternating-current  work  the  number  of  poles  is 
halved  by  reversing  the  terminals  of  any  two  successive  field  coils,  skip- 
ping the  two  coils  and  reversing  the  next  two.  This  being  most  con- 
veniently done  by  changing  the  connections  at  the  starting  switch.  This 
change,  together  with  the  "long  coil"  winding  and  the  diagram  of  con-" 
nections  for  four  poles  fits  the  machine  for  operating  at  60  cycles.  The 
speed  will  now  be  1800  r.p.m.  and  the  voltage  and  output  practically  the 
same  as  before. 

The  following  is  a  brief  summary  of  the  general  dimensions  and  data 
for  winding : 

Eight-pole  machine:  1800  r.p.m.  at  60  cycles;  1875  r.p.m.  at  125 
cycles.  Armature  $l/>  ins.  diameter,  4  ins.  long,  48  slots  3-16  in.  X  /^  in. ; 
3-16  in.  =  188  in.  width  of  slot,  allowing  three  No.  17  wires  and  insulation 
of  18  mils;  5  in.  depth  slot,  taking  eight  No.  17  wires  and  insulation  and 


FOUR   KILOWATT  DOUBLE-CURRENT  MACHINE 


bands  of  45  mils.,  giving  24  conductors  per  slot.    Total,  1,152  conductors. 
Direct-current  output  at  470  circ.  mils  per  ampere  =  36  amps. 

There  will  be  455  ampere-turns  in  each  field  coil.    Mean  length  of  I 


turn  =  13.5  inches  circ.  mils  shunt  wire  = 


_  ii  X  13-5  X  455  _ 


=  465- 


12    X    12 

Use  No.  23,  having  509  circ.  mils,  .034  in.  diam.,  d.c.c.  ;  620  turns 
(approx.)  per  coil,  in  14  layers,  44  turns  wide.    Wind  in  two  sections  and 
bring  out  individual  terminals  from  each  section. 
Resistance  of  shunt  field  =  8  X  620  X_i3:5_>Og:3  •_  1I4  ohms 

I  ,OOO    /\    1  2 


Normal  shunt  current  =  -—  =  75  amP-    Use  rheostat  of  about  40 

ohms  in  shunt  field.    Weight  of  wire  in  shunt  coils  : 

8X620X  13-5  X  i.54=  g    "lb 

1,000   X    12 


Total  length  of  wire  on  armature  : 
4«  X  .8  X 


12 

Total  weight  of  wire  on  armature : 

864  X  6.2 

1,000 
Resistance  of  armature : 

1,000  X  64 


=  5-37 


•=  .068  ohms. 


864  X  5-04 
Drop  in  armature  at  full  load : 

36  X  .068  =  2.45  volts. 


Part 

Material 

Total 

lines 

Cross- 
section 

B. 

H. 

L. 

Ampere 
turns 

I  armature.. 

Wrought  iron 

170,000 

4.     in. 

42.500 

4 

1.5 

6 

2  air  gaps  .  . 

Air 

340,000 

6. 

56,600 

17,880 

.0313 

566 

5  teeth  

Wrought  iron 

340,000 

•3. 

113,000 

1  08 

.5 

54 

Wrought  iron 

420,000 

5. 

85.000 

18 

1.7 

31 

Cast  iron 

210,000 

4.7] 

42,000 

68 

•2.7 

252 

Total  ampere-turns 909 


CHAPTER  XIII. 


SINGLE-PHASE    RECTIFIER. 


The  accompanying  drawings  and  description  constitute  a  design  for 
a  machine  to  "rectify"  single-phase  alternating  current;  that  is,  to  change 
it  into  a  pulsating  direct-current  without  changing  its  e.m.f. 

Fig.  134  shows  an  end  view  of  the  field  magnet  with  coils  in  place, 


FIG.    134. — END  ELEVATION  OF  RECTIFIER  FIELD  MAGNET. 

two  of  which  are  represented  as  cut  away  to  show  the  shape  of  the  poles 
and  cores.  The  circular  yoke,  poles  and  arms  to  support  the  bearings 
are  cast  in  one  piece,  thus  avoiding  joints  in  the  magnetic  circuit,  and  re- 
ducing the  machine  work  to  a  minimum.  The  pattern  for  the  field  casting 


SINGLE-PHASE  RECTIFIER 


should  be  made  in  two  pieces,  the  parting  being  made  along  the  line  AB 
in  Fig.  135. 

The  field  casting  is  mounted  on  the  face  plate  of  the  lathe — one  of  9 
ins.  swing  will  do — and  the  poles  are  bored  out  to  the  required  diameter, 
2l/2  ins.,  at  the  same  time  the  arms  for  the  bearings  are  finished  to  the  arc 
cf  a  circle  5T4  ins-  in  diameter.  These  may,  however,  be  filed  and  fitted 
by  hand,  if  this  is  more  convenient,  but  the  method  of  boring  is  the  more 
accurate. 

Fig.  137  shows  a  section  through  the  armature,  commutator  and 
bearings.  The  armature  core  is  built  up  of  laminated  iron  in  the  usual 
way,  and  held  together  by  heavy  washers  of  wrought  iron  threaded  on 
the  shaft  at  either  end.  A  cast-iron  core  may  be  used,  though  it  will,  of 
course,  heat  up  more  than  the  laminated  one. 

The  commutator  and  collector  rings  are  preferably  made  of  copper, 
though  brass  may  be  used.  A  piece  of  ingot  copper  can  be  obtained  and 


Ei.CC. 


FIG.    135.  —  SIDE   ELEVATION   OF   RECTIFIER  FIELD  MAGNET. 

forged  into  a  circular  blank  suitable  for  turning  up  into  rings  and  a  com- 
mutator. The  commutator  is  turned  as  a  solid  cylinder  of  the  required 
section,  and  it  is  then  cut  into  four  equal  segments  with  a  milling  ma- 
chine, or  with  a  sharp  hacksaw  and  hand  power,  if  a  milling  machine  is 


n8 


ELECTRICAL  DESIGNS 


not  available.  The  segments  are  then  built  up  with  mica  insulation  and 
clamped  in  a  brass  sleeve  fitting  the  shaft. 

It  is  well  to  follow  the  design  of  arc  machine  commutators  to  some 
extent  and  to  allow  about  1-16  in.  air  insulation  between  the  segments  at 
the  top.  Thus  the  mica  insulation  will  not  be  injured  if  sparking  occurs. 
Oil  and  copper  dust  should  not  be  allowed  to  accumulate  in  the  air  spaces 
thus  formed,,  as  this  would  cause  a  severe  short  circuit. 

Connections  are  made  between  the  collector  rings  and  commutator 
bars  with  some  strips  of  copper  about  1-32  in.  thick  and  y%  in.  wide,  laid 
in  the  armature  slots,  which  have  been  made  about  1-16  in.  wider  than 


K^ 


/T;          .' 

H — H—  a--T— I 


*  •: 


I 
u. 


-r- 


W^ 
V  i  i 


«0|1D 


.1 

10 


'3 


i  S 


iJt 


3 


FIG.    136. — PLAN  VIEW  OF  RECTIFIER  FIELD  MAGNET. 

would  otherwise  be  necessary  in  order  to  accommodate  both  the  coils 
and  the  connection  strips.  The  back  collector  ring  is  drilled  with  %  in. 
holes  at  four  equidistant  points,  two  of  these  holes,  the  opposite  ones,  go 
through  the  ring  and  part  way  into  the  front  ring,  the  other  two  holes  are 
drilled  only  part  way  in.  Some  short  pieces  of  copper  wire,  about  No.  10, 
are  soldered  one  into  each  hole,  the  two  wires  from  the  front  ring  being 
insulated  from  the  other  ring  where  they  pass  through  it.  The  four  wires 
are  soldered  one  to  each  strip,  and  these  carry  the  current  across  the 
armature  and  connect  to  the  commutator  segments. 


SINGLE-PHASE  RECTIFIER 


119 


120 


ELECTRICAL   DESIGNS 


Diametrically  opposite  segments  of  the  commutator  are  thus  con- 
nected to  the  same  collector  ring,  and  neighboring  segments  have  be- 
tween them  the  whole  potential  difference  of  the  alternating  circuit.  It 
is  much  better  not  to  connect  the  copper  strips  permanently  to  the 
commutator  until  the  builder  has  decided  where  he  wishes  to  place  the 
brushes ;  the  commutator  may  then  be  twisted  around  on  the  shaft  to  the 
correct  position  and  the  connections  made  permanently.  The  brushes 
may  be  placed  wherever  they  will  be  most  convenient,  the  only  restriction 
being  that  they  must  be  9.0°  apart  and  must  pass  from  one  segment  to  the 
next  at  the  same  instant  that  an  armature  tooth  is  exactly  under  a  pole. 

Fig  138  shows  an  armature. coil  and  the  method  of  placing  the  coils 
upon  the  core.  The  coils  are  wound  on  a  form,  and,  after  being  taped, 


I 

w 

_ 

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r      ) 

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\£ 

1 

i 

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,  i  i    i 

H • 


PJC.BROSH    HOU.PCR. 

•     TWO    «F  THtst  . 


ftC.  BROSH  HOUDE.R  . 

TWO    Of  THfcSfc. 


Ru. 

Of 

-AMER.  CLEC. 


FIG.    141.—  DETAILS-  OF  JOURNAL  YOKES,    BRUSH   COLLAR  AND  BRUSH-HOLDERS. 

are  slipped  over  the  top  of  a  tooth.  The  slack  is  then  taken  up  by  bend- 
ing the  ends  down  in  a  semi-circular  shape  and  fastening  them  in  this 
position  by  screws  which  carry  small  fibre  or  hardwood  bushings, 

In  addition  to  this  the  armature  should  be  banded  at  one  or  two 
points  with  No.  28  brass  or  German  silver  wire,  small  notches  having 
been  turned  in  the  core  to  receive  the  bands  and  allow  them  to  come  flush 
with  the  surface  of  the  core. 


SINGLE-PHASE  RECTIFIER  121 

With  an  iron-clad  armature  like  this,  the  clearance  need  not  be 
more  than  from  1-64  in.  to  1-32  in. ;  just  how  much  it  will  be  depends 
somewhat  on  the  builder's  skill ;  1-64  in.  clearance  has  been  indicated  on 
the  drawings,  and  with  care  taken  in  adjustment  it  should  not  be  difficult 
to  obtain  this  figure.  Fig.  139  is  an  end  view  of  the  rectifying  commu- 
tator. Fig.  140  is  a  flywheel  pulley  which  it  will  be  found  advisable  to 
use  in  order  to  obtain  smooth  running. 

Fig.  141  shows  the  bearing,  brush  yoke  and  brush  holders.  The 
bearings,  while  not  so  simple  in  construction  as  some  other  designs, 
have  proven  very  satisfactory.  The  rings  carry  up  a  plentiful  supply  of 
oil,  and  what  runs  out  at  the  end  returns  to  the  well  and  does  not  fly  off 
outside  the  bearing  and  spatter  the  surroundings  with  grease  spots. 

In  finishing  the  bearings  the  cap  is  first  fitted  and  fastened  by  two 
machine  screws.  The  bearing  is  then  placed  in  the  chuck  and  bored  out 
to  a  diameter  of  y&  in.  clear  through.  At  the  same  time  the  ends  are 
turned  off  534  ins.  in  diameter  to  fit  the  arms  on  the  field  casting,  and 
the  outside  of  the  boss  is  turned  off  i><  in.  diameter  where  it  is  to  receive 
the  brush  yoke.  The  sleeve  which  forms  the  bearing  proper  is  turned  a 
tight  fit  for  the  central  part  of  the  box,  then  when  the  cap  is  screwed 
down  it  will  be  held  firmly  in  place.  The  grooves  for  oil  rings  can  be 
cut  in  the  sleeve  conveniently  by  mounting  it  eccentrically  in  the  chuck 
and  using  a  thin  cut-off  tool. 

The  brush  yoke  is  shown  with  two  arms  90  deg.  apart.  Another 
pair  of  arms  and  brushes  might  be  added  if  it  is  desired  to  have  more 
current  carrying  capacity. 

The  direct  current  brushes  had  better  be  larger  than  the  alternating' 
current  brushes,  and  the  holders  should  have  spring  tension,  unless  a 
very  springy  brush  is  used.  Copper  brushes  are  better  for  this  purpose 
than  carbon,  as  they  make  better  contact  and  cause  less  sparking. 

Figs.  142  and  143  show  the  field  and  armature  coils  respectively,  with 
the  forms  upon  which  they  are  wound.  The  former  is  best  made  of  hard 
wood,  and  consists  of  a  block  and  two  flanges,  all  held  together  by  two 
wood  screws  and  having  a  ]A  in.  hole  through  the  center  for  placing  on 
a  mandrel  in  the  lathe.  The  block  should  be  made  a  trifle  larger  than 
the  pole  over  which  the  finished  coil  is  intended  to  go,  and  it  should  be 
given  a  slight  taper  of  about  1-16  in.,  so  that  it  can  be  readily  slipped  out 
of  the  finished  coil. 

Before  beginning  the  winding  a  short  piece  of  tape  is  laid  in  the  long 
sides  of  the  former,  with  the  ends  left  sticking  out.  When  the  form  is 
wound  full  these  pieces  of  tape  are  tied  tightly  over  the  coil,  and  will 
hold  it  in  shape  while  the  former  is  taken  apart  and  the  coil  is  receiving 


122 


ELECTRICAL  DESIGNS 


its  wrapping  of  tape.  After  being  shellacked  and  dried,  the  coil  is  placed 
on  the  poles  and  hard  wood  wedges  driven  in  between  coil  and  pole,  thus 
holding  it  securely  in  place. 

The  same  form  may  be  made  to  serve  for  both  field  and  armature 
coils,  if  the  field  coils  are  wound  first,  and  then  the  block  reduced  in 
thickness  from  ^J  in.  to  y%  in.,  the  armature  coils  having  the  same  inside 
dimensions  as  the  field  coils,  but  being  only  half  as  thick. 

The  fields  are  wound  with  No.  28  double  cotton  covered  wire  and 
the  armature  with  No.  23.  If  the  coils  are  wound  to  the  specified  di- 
mensions they  will  have  nearly  enough  the  required  number  of  turns. 

Fig.  144  shows  a  diagram  of  connections  and  Fig.  145  some  e.  m.  f. 
curves.  For  100  volts  the  field  and  armature  coils  are  connected  four 
in  series,  as  shown.  For  50  volts  the  coils  may  be  connected  two  in 
series  and  the  twos  in  multiple.  The  armature  terminals  are  tapped  onto 


f  I  tLP.  BOBBIN. 

Wm»  VIP  TO  sue. 


c 


WOOD    Ftosnta. 
FOR     FIELD. 


FIG.    142. — A  FIELD  COIL  AND  THE 
WINDING  FRAME. 


Woo  9 

FORi-lfi-R. 
AMER.    C1.CC. 


FIG.    143. — AN  ARMATURE   COIL  AND   THE 
WINDING  FRAME. 


COIL. 
Mart:    WiNP  UP  TO  eiz& 
WITH  *2.3  P.CC 


tHe  collector  rings,  or  what  amounts  to  the  same  thing,  placed  across  any 
two  successive  commutator  segments. 

The  connections  on  both  armature  and  field  should  be  such  as  to 
produce  alternate  north  and  south  polarity  all  the  way  round.  If  all  the 
coils  have  been  wound  in  the  same  direction  and  placed  on  the  poles  the 
same  way,  connect  beginning  to  beginning  and  ending  to  ending,  and 
the  polarity  will  be  right.  The  machine  will  run  at  1,800  r.  p.  m.  on  a  60 
cycle  circuit,  and  on  a  I25~cycle  circuit  it  will  have  to  make  3,750  r.  p.  m. 
This  it  can  easily  do  if  the  armature  is  well  balanced  as  it  should  be. 

Since  the  strength  of  the  field  has  a  considerable  effect  on  the  be- 


SINGLE-PHASE  RECTIFIER 


123 


havior  of  a  synchronous  motor,  it  is  best  to  have  an  adjustable  resistance 
in  the  field  circuit  of  this  machine,  so  that  the  field  can  be  adjusted  until 
the  minimum  armature  current  is  obtained. 

Referring  now  to  the  curves  in  Fig.  145,  it  is  clear  that  if  the  rectifier 
is  running  in  synchronism  and  the  angular  position  of  the  brushes  is  cor- 
rect, the  brushes  will  pass  from  segment  to  segment  at  the  instant  when 
the  e.  m.  f.  curve  reaches  its  zero  value  at  the  points,  a,  a,  a,  etc.  As  the 
brushes  in  passing  from  segment  to  segment  overlap  two  segments  for  a 
brief  interval,  they  form  a  dead  short  circuit  on  the  alternating  current 
mains  during  the  interval.  This  will  not,  however,  result  in  any  damage 
if  the  e.  m.  f .  becomes  zero  at  the 
same  instance.  If,  however,  the 
brushes  had  been  incorrectly  placed 
and  commutation  occurred  at  the 
points  b,  by  b,  etc. ,  an  e.  m.  f .  of  value 
equal  to  the  ordinate  at  b  would 
be  short  circuited  four  times  in  a 
revolution,  and  serious  sparking 
would  result. 

This  state  of  affairs  is  easily 
remedied  by  shifting  the  brushes, 
which  corresponds  to  changing  the 
angular  position  of  the  point  of 
commutation  until  a  position  such 
as  a  a  is  reached,  when  all  sparking 
will  disappear.  If  the  armature 
falls  out  of  step,  or  if  it  is  thrown 
into  circuit  before  complete  syn- 
chronism is  reached,  a  short  circuit 
travels  over  every  portion  of  the  e. 
m.  f.  wave,  at  a  slow  rate  equal  to 
the  difference  between  synchronous  speed  and  the  actual  speed  at  that 
instant,  the  result  being  a  magnificent  display  of  fireworks  and  probably 
a  fuse  blown. 

To  obviate  this  latter  difficulty  a  resistance  or  choking  coil  should  be 
placed  in  series  with  the  alternating  current  end  at  the  moment  of  start- 
ing, and  cut  out  when  it  is  seen  that  the  machine  has  settled  down  to 
steady  running.  Such  a  resistance  will  not  have  any  appreciable  effect 
on  the  small  current  drawn  by  the  armature  and  field  windings,  but 
should  be  of  such  a  value  as  to  limit  the  current  to  about  10  amperes, 
should  a  short  circuit  occur. 


FIG.    144. — RECTIFIER   CONNECTIONS. 
FIG.   145. — SOME   E.  M.  F.  CURVES. 


I24 


ELECTRICAL   DESIGNS 


The  ordinary  method  of  a  synchronizing  lamp  is  not  easily  applicable 
here  on  account  of  the  small  size  of  the  machine ;  and,  moreover,  a  little 
practice  will  enable  the  operator  to  judge  by  ear  the  proper  instant  for 
closing  the  circuit. 

Thus  by  making  slight  changes  in  the  connections,  as  already  pointed 
out,  this  machine  may  be  used  as. a  rectifier  on  single-phase  circuits  of  50 
or  100  volts  and  60  or  125  cycles.  The  amount  of  rectified  current  which 
may  be  drawn  is  not  limited  in  any  way  by  the  horse-power  capacity,  but 
will  generally  be  limited  only  by  the  capacity  of  the  transformer  which  is 
supplying  the  current.  Thus  from  50  to  100  amperes  may  be  drawn,  de- 
pending somewhat  on  the  nature  of  the  load  into  which  the  rectifier  is 
feeding  current. 

The  machine  may  also  be  used  as  a  self-exciting  synchronous  motor, 


FIG.    146. — END   AND   SIDE   VIEWS    OF  THE   COMPLETE   MACHINE. 

developing  from  i-io  to  %  horse-power,  according  to  the  strength  of 
field  ;  and  finally  it  may  be  driven  by  belt  as  a  self-exciting  alternator,  sup- 
plying either  an  alternating  current  or  a  rectified  direct  current,  or  both, 
up  to  about  100  watts  output. 

Fig.  146  was  reproduced  from  photographs  illustrating  a  rectifier 
built  from  these  designs. 


CHAPTER  XIV. 


UNIVERSAL  ALTERNATOR  FOR  LABORATORY  PURPOSES. 


The  design  of  the  machine  illustrated  in  the  accompanying  engrav- 
ings was  adopted  for  the  following  reasons: 

i.  Simplicity  of  construction  by  students  in  the  engineering  shops, 
without  special  tools  or  dies;  2.  Its  similarity  to  a  bi-polar  dynamo  so  as 
to  illustrate  one,  two  or  three-phase-current  generation,-  but  without  3 


F— 


r 


uJ- 


-4K* 


Am.Elfc. 


FIG.  147. — JOURNAL  PEDESTALS  AND   BOXES. 


low  limit  to  the  frequency;  3.  To  illustrate  practically  the  effect  of  com- 
bining e.  m.  fs.  differing  in  phase,  in  a  variety  of  ways.  • 
To  accomplish  these  objects,  both  the  field  and  the  armature  were 
made  with  poles,  the  latter  having  two  more  than  the  former.     The  field 


126 


ELECTRICAL   DESIGNS 


revolves  and  is  of  the  C.  E.  L.  Brown  type.  The  armature  is  made  of 
sheet-iron  rings  held  together  by  bolts  between  cast-iron  plates.  The 
spaces  between  the  poles  of  the  armature  were  milled  out  after  the  rings 
had  been  bolted  together.  The  field  is  made  of  two  identically  similar 
steel  castings,  each  with  five  poles  symmetrically  spaced  and  pointing  in 
the  same  direction  parallel  to  the  axis.  The  field  has  thus  ten  poles, 
alternating  in  sign,  and  the  armature  twelve.  The  following  are  some  of 
the  dimensions : 

Diameter  of  armature  pole-faces,  ioj4  ins. ;  length  of  faces  parallel 
to  shaft,  4  ins.;  width  of  pole-faces,  iy2  ins.;  pitch  of  poles  on  armature, 
2.68  ins. ;  depth  of  poles,  JX  in. ;  net  cross-section  of  pole  in  square 
inches,  5.4.  The  armature  poles  have  forty  turns  of  No.  16  wire  each. 


Am.Eled. 


FIG.  148. — SECTION  OF  ONE 
MAGNET  POLE. 


FIG.  151. — COMPLETE   MACHINE  WITHOUT  BASE. 


The  double  air  gap  is  %  in.     The  field  coil  contains  1012  turns  of  No.  16 
double-cotton-covered  wire. 

The  armature  coils  were  wound  in  reverse  order  from  pole  to  pole 
in  the  usual  way.     They  are  connected  in  pairs,  and  the  terminals  of  each 


UNIVERSAL  ALTERNATOR  FOR  LABORATORY  PURPOSES        127 

pair  are  brought  up  to  binding  posts  on  a  board  bolted  to  the  top  of  the 
machine. 

The  machine  will  give  i ,  500  watts  when  connected  in  three-phase 
zig-zag  mesh  fashion  and  driven  at  i ,  650  r.  p.  m. 

The  completed  machine  is  shown  in  Fig.  151,  although  the  engraver 
has  left  off  the  base-plate  on  which  the  armature  and  pedestals  are 
mounted. 


An  inspection  of  the  diagram  (Fig.  152)  will  show  that  diametrically 
Opposite  poles  of  the  field  are  of  opposite  sign,  while  the  corresponding 
coils  of  the  armature  are  similarly  wound.  Hence  with  a  closed-coil 


128 


ELECTRICAL  DESIGNS 


armature  the  e.  m.  fs.  balance  exactly  as  with  a  bi-polar  dynamo.  If, 
therefore,  connection  be  made  with  the  armature  at  two  opposite  points, 
the  current  in  the  external  connecting  circuit  will  be  alternating.  Further, 
two  such  circuits  connected  at  right  angles  will  convey  currents  in  quad- 
rature. By  connecting  at  points  120  degs.  apart,  three-phase  currents 
will  be  obtained. 

Again,  the  coils  may  be  joined  either  in  mesh  or  star  fashion  by 
means  of  the  binding  posts  at  the  top,  and  we  may  zig-zag  across  either 
\vith  two  or  three-phase  connections,  so  as  to  connect  opposite  coils  by 


FIG.   152. — SIDE   ELEVATION    OF   THE   MACHINE. 

twos  or  threes  with  no-phase  difference  between  the  two  opposite  groups. 
This  connection,  of  course,  gives  the  highest  e.  m.  f. 

It  is  evident  that  the  phase  difference  from  coil  to  coil  is  30  degs.  or 
one-twelfth  of  a  period.  Hence  the  voltage  for  a  given  magnetic  flux  cut 
per  second,  calculated  in  the  usual  way,  must  be  first  divided  by  V2  to 
reduce  from  maximum  to  virtual  volts,  and  then  the  equal  e.  m.  fs.  gen- 


UNIVERSAL  ALTERNATOR  FOR  LABORATORY  PURPOSES        129 


crated  by  the  several  coils  must  be  added  geometrically  with  a  phase  dif- 
ference of  30  degs.  from  coil  to  coil.  Since  the  e.  m.  fs.  of  the  several 
coils  are  equal  and  differ  in  phase  by  one-twelfth  of  a  period,  the  series 
may  be  represented  by  a  regular  polygon  of  twelve  sides  (Fig.  150). 
Hence,  if  E  be  the  e.  m.  f.  of  one  coil,  the  following  will  be  the  e.  m.  fs. 
of  the  several  groups  of  coils  : 

AC,  E.  M.  F.  of  two    coils,  2  E  cos  15°  =  1.93  E. 

AD,  "  "three  "  E  -f  2  Ecos  30°  =  2.73  E. 

AE,  "  "  four  "  2  E  (cos  15°  -f  cos  45°)  ==  3.346  E. 

AF,  "  "five  "  E  -f-  2  E  (cos  63°  +  cos  30°)  =  3.73  E. 
AC,  "  "six  "  4£  cos  15°=  3.86^. 

The  phase  difference  between  the  coils  reduces  the  e.  m.  f.  of  the 
six  in  series  on  either  side  to  3.86  -f-  6  or  0.643  °f  what  it  would  be  were 
there  no  such  phase  difference. 


S  '§  g 


Volts  i-   i- 

g  §  3  g  S  §  S 


G  Am.Elec. 

FIG.  ISO. — DIAGRAM   OF   E.  M.  FS. 


FIG.  153. — MAGNETIC   CURVE  AND   EX- 
TERNAL   CHARACTERISTICS. 


It  will  be  seen  from  the  subjoined  table  that  the  observed  e.  m.  fs. 
agree  very  closely  with  those  computed  from  the  foregoing  equations. 


One  coil.  . . 
Two  coils.  . 
Three  coils. 
Four  coils  . 
Five  coils. . 
Six  coils  . . 


Observed. 
21 

40.3 
55-5 

.  'Vi     69 
75-5 

•          77-5 


Computed, 
20.5 
39-6 
56 
63.6 
76.5 
79.1 


With  about  2000  ampere-turns  on  the  field  coil,  the  following  ob- 
served voltages  were  obtained  for  the  several  connections  described  in 


130  ELECTRICAL  DESIGNS 

the  first  column.     The  computed  values  are  readily  obtained  from  the 
preceding  expressions. 

Connection.  Observed.  Computed. 

Two-phase  mesh 79  79 

"         "      star 112  112 

Three-phase  mesh 69  68. ( 

"         "      star 121  nS.S 

"         "      zig-zag 136  136.8 

The  alternator  was  driven  by  a  motor  on  a  power  circuit  and  the 
voltage  varied  a  good  deal.  Some  of  the  irregularities  of  voltage  in  the 
generator  are  accounted  for  by  the  variation  in  speed  of  the  motor. 

Fig.  153  shows  the  curve  of  magnetization  and  the  characteristic  with 
3  amperes  in  the  field.  The  armature  was  connected  as  a  closed  coil,  and 
only  a  single  alternating  current  was  drawn  from  it.  The  total  drop  for 
full  load  is  ii  volts;  of  these,  about  4.5  volts  are  due  to  drop  in  the  arma- 
ture, and  the  rest  must  be  set  down  to  self-induction. 


CHAPTER  XV. 


ONE-QUARTER  HORSE-POWER  SINGLE-PHASE   INDUCTION  MOTOR. 


The  induction  motor  described  in  this  article  was  designed  to  be 
built  by  amateurs,  and  the  aim  has  been  to  make  it  simple  and  easy  to 
construct.  It  is  designed  for  a  single-phase  alternating  circuit  of  104 
volts  and  a  frequency  of  60  cycles  per  second.  It  has  four  poles,  and, 
therefore/  its  synchronous  speed  would  be  1,800  revolutions  per  minute. 
The  actual  speed  of  the  motor  at  load  will  be  about  10  per  cent,  less  than 
the  synchronous  speed. 

The  primary  or  stator  has  a  plain  ring  winding,  and  the  secondary 
or  rotor  a  so-called  squirrel-cage  winding,  consisting  of  bare  copper 
conductors,  embedded  without  insulation  in  an  iron  core,  all  conductors 
being  connected  at  the  ends.  The  bearing  supports  are  provided 
with  an  oil  chamber,  and  either  a  ring  or  felt  self-oiler  may  be  used. 

In  making  the  calculations  for  the  motor  we  will  follow  the  method 
much  used  in  transformer  calculation,  which  consists  in  assuming  the 
various  losses,  and  from  these  losses  determine  the  dimensions  of  the 
parts  in  which  they  occur.  We  will  set  down-  the  efficiency  of  our  ma- 
chine at  60  per  cent,  and  the  power  factor  at  75,  figures  which  obtain  in 
machines  of  this  size  on  the  market.  The  output  being  J4  horse-power  or 

1 86  watts,  the  intake  will  be — 2- —  =  310  watts,  and  the  total  losses  are, 

therefore,  124  watts.    We  will  make  a  preliminary  division  of  this  loss  as 
follows : 

Primary  C'R  loss,  25  watts. 

Secondary  C2R  loss,  15  watts. 

Hysteresis,  40  watts. 

Friction  and  eddy  currents,  34  watts. 

The  primary  current  at  full  load  will  be ~- =  4  amperes. 

From  fne  C'R  loss  and  the  current  strength  we  may  now  find  the  resist- 
ance of  the  primary  circuit,  R  =-^|-=  1.56  ohms. 

4 


132  ELECTRICAL   DESIGNS 

Allowing  a  current  density  in  the  primary  winding  of  2,000  amperes 
per  sq.  in.,  we  find  that  the  wire  to  be  used  is  No.  16  B  &  S.  The  length 
of  this  wire,  which  will  have  a  resistance,  when  warm,  of  1.56  ohms,  is 

354  ft. 

The  ohmic  component  of  the  e.m.f.  in  the  primary  circuit  is 
1.56x4  =  6.24  volts,  and  the  induction  or  counter  e.  m.  f.,  therefore 

V  104' +  6.24--*  X  104  X  6.24  X. 75  =  9942  volts' 

In  a  single-phase  induction  motor  the  strength  of  the  rotating  field 
is  not  constant,  but  fluctuates.  We  will  base  our  calculations,  however, 
on  an  equivalent  rotating  field  of  constant  strength. 

Fig.  154  shows  a  diagram  of  the  magnetic  field  in  the  motor.  Let  O 
be  the  flux  of  magnetism  that  passes  through  the  teeth  of  the  stator,  be- 
tween A  and  B.  Then,  as  the  magnetic  field  rotates  with  a  velocity  of 


FIG.    154. — MAGNETIC  CIRCUIT. 


FIG.    155. — DIAGRAM   OF   STATOR  WINDING. 


1,800  r.p.m.,  and  the  conductors  are  stationary,  each  conductor  cuts  this 
number  of  lines  120  times  per  second.     The  mean  value  of  the  e.  m.  f . 

^>  \/   I  2O 

induced  in  one  conductor  will  be  —  -— — ,  and  the  square  root  of  the 


mean  square, 


i  2O  \f 


ioc 


—  .  But  this  value,  multiplied  by  the  number  of 


conductors,  is  the   maximum   e.m.f.  induced  in  the   primary  winding1. 
Representing  by  n  the  number  of  cycles  per  second  (60),  we  may  write : 

(0 


10' 


We  will  allow  a  maximum  magnetic  density  in  the  stator  core  of 
25,000  lines  per  sq.  in.,  and  make  the  axial  width  of  the  core  double 


ONE-QUARTER   HORSE-POWER  INDUCTION  MOTOR  133 

its  radial  depth.     The  maximum  magnetic  flux  at  any  cross  section  of 

the  core  is — -.    The  area  of  the  cross  section  will,  therefore,  be * 

2  50000 

The  periphery  of  this  area  is  6A  —       —and  taking  the  length  of  turn  10 

100000 

per  cent,  greater  than  the  periphery  of  the  core,  we  have  for  it  the  value, 


i ooooo 

The  number  of  conductors  is  equal  to  the  total  length  of  wire  divid- 
ed by  the  length  of  one  turn : 

L 


6.6 


I OOOOO 

By  substituting  this  value  of  C  in  equation  (i),  transforming  and  re- 
ducing, we  obtain. 

i.8^2.io12 
n2  Lz 

We    may    no>w    substitute     as    follows :      E    =    99.4,    n     =    60, 
L  =  4,248  (inches).    This  gives  3>   =  275,000. 

The  magnetic  cross  section  of  the  field  core  is •  =  5.5  sq.  in., 

50000 


/ 
,  6.6  \ 


and  the  length  of  one  turn,  6.6  \ —      —  •=.  nins. 

i ooooo 

The  number  of  conductors  is — —    -  =  386.       As  we  have  four 

poles,  the  number  of  conductors  should  be  divisible  by  four,  and  we  will, 
therefore,  take  384  conductors. 

These  conductors  may  be  distributed  in  32  slots,  giving  12  con- 
ductors per  slot.  We  will  wind  these  conductors  2  wide  and  6  deep. 
The  diameter  of  No.  16  double  cotton-covered  magnet  wire  is  61  mils. 
For  insulation  we  allow  30  mils  on  each  side  of  the  slot  and  30  mils  at 
the  bottom ;  also  a  clearance  space  of  30  mils  below  the  surface  of  the 
teeth.  This  gives  for  the  dimensions  of  the  slots  a  depth  of  426  mils  and 
a  width  of  182  mils. 

The  cross  sectional  area  of  the  core  was  5.5  sq.  ins.,  and  the  ratio  of 
axial  length  to  radial  depth,  100.  We  will  make  the  core  3^  ins.  long 
by  i  9-16  ins.  deep,  which  approximately  satisfies  the  above  two  condi- 
tions. 

The  mean  magnetic  density  in  the  teeth  may  be  taken  at  32,000  lines 
per  sq.  in.    There  are  in  all  550,000  lines,  each  passing  through  the  teeth 


134  ELECTRICAL  DESIGNS 

twice,  which  is  equivalent  to   1,100,000  passing  once.     The  magnetic 

cross  section  of  the  teeth  should,  therefore,  be—  —  =  344  sq.  ins. 

32,000 

Dividing  this  area  by  3.5  ins.  the  length  of  the  teeth,  we  obtain  9.83  ins., 
the  circumferential  space  taken  up  by  the  teeth.  The  circumferential 
space  taken  up  by  the  slots  is  32  +  .182  =  5.82  ins.  The  total  inner  cir- 
cumference of  the  stator  is,  therefore,  15.65  ins.,  and  the  diameter  practi- 
cally 5  ins. 

Some  of  the  lines  of  force  that  pass  through  the  primary  circuit  do 
not  enter  the  secondary  circuit,  but  leak  around  it.  The  leakage  co-effi- 
cient is  about  1.2.  The  total  lines  of  force  crossing  the  gap  number, 

therefore,  -  -  =  916,600.    By  dividing  this  number  by  the  surface 

of  the  gap  we  obtain  the  mean  magnetic  density  in  it,—  — •=  16,600. 

jj 

The  output  of  our  motor  is  186  watts,  and  the  allowance  for  friction 
30  watts.  The  total  energy  transformed  from  the  electrical  to  the  me- 
chanical state  is,  therefore,  216  watts.  The  C2R  loss  in  the  secondary 
is  15  watts.  It  is  a  well-known  fact  that  the  ratio  of  the  motor  speed  to 
the  speed  of  synchronism  is  the  same  as  the  ratio  between  the  energy 
transformed  in  the  secondary  to  the  energy  absorbed  by  it.  The  speed 

of  our  motor  at   load  will   be,  therefore,     — —      1800  =  1680  r  p.  m 

231 

When  a  rotor  of  5  ins.  diameter,  running  at  1,680  r.  p.  m.  develops 
mechanical  energy  at  the  rate  of  216  watts,  the  tangential  force  on  its 
circumference  is  4. 34  Ibs.  Now  a  conductor  a  inches  in  length,  carrying 
an  alternating  current  whose  I/  mean  square  value  is  I  in  a  sinusoidal 

field  of  mean  intensity,  B,  has  a  mean  force  of  Ibs.    exerted 

10,180,000 

/"*  7"  ft    7? 

on  it.     If  their  be  C  conductors,  the  force  is  —  Ibs.      We  may 

10,180,000 

substitute  the  values  of  a  and  B  and  equate  this  expression  to  our  cir- 

.         ...  C/X  3.5  X  16,600 

cumferential  force,   • —  =  4.34, 

10,180,000 

Hence,    +34  X  10,180000  =     -^ 
3.5  X  16,600 

The  number  of  secondary  conductors,  multiplied  by  the  amperes 
per  conductor  is,  therefore,  760.  We  will  put  15  conductors  on  the 
secondary,  which  will  give  a  current  of  a  little  over  50  amps,  per  con- 
ductor. The  loss  in  each  conductor  is  one  watt,  and  the  resistance  per 

conductor,  therefore, ohm.    This  includes,  of  course,  the  resistance 


ONE-QUARTER    HORSE-POWER  INDUCTION  MOTOR  135 

of  the  soldered  joints  and  the  return  on  the  ends,  which  cannot  be  ex- 
actly calculated,  but  which  may  be  taken  at  one-half  of  the  total,  so  that 

the  actual  resistance  of  the  conductor  is  only—     -  ohm.     The  size  of 

5000 

wire  of  which  a  length  of  33/2  ins.  has  a  resistance  of  —  -  —  ohm  is  No. 


8  B.  &  S.,  which  we  will  use  for  the  cage  winding  of  the  motor.  The 
maximum  density  of  our  equivalent  rotating  field  of  constant  strength 
was  25,000  lines  per  square  inch.  We  have  to  make  a  small  allowance 
for  the  insulation  of  the  discs,  and  also  take  into  account  the  fact  that 
our  actual  rotating  field  is  fluctuating,  which  subjects  parts  of  the  core 
to  a  considerably  higher  magnetic  density.  At  35,000  lines  per  square 
inch  the  hysteresis  loss  per  cubic  foot  at  60  cycles  is  360  watts.  There 
are  129  cubic  inches  of  iron  in  the  stator  core  below  the  teeth,  and  the 
hysteresis  loss  would,  therefore  ,  be  28  watts.  In  the  teeth  the 
magnetic  density  will  reach  60,000  lines  per  square  inch.  The  hysteresis 
loss  at  this  density  and  frequency  is  840  watts  per  cu.  ft.  There  are  17 
cu.  ins.  of  iron  in  the  teeth,  and  the  hysteresis  loss  is,  therefore,  8.5 
watts.  In  the  rotor  the  hysteresis  loss  is  practically  nil,  as  the  frequency 
of  the  reversal  of  magnetism  is  proportional  to  the  slip  of  the  rotor, 
which  at  full  load  is  only  one-fifteenth  the  frequency  of  reversal  of  the 
magnetism  in  the  stator  core.  Our  total  hysteresis  loss  is,  therefore, 
36.5  watts,  which  is  well  within  the  limit  of  our  allowance  for  it. 

CONSTRUCTION  OF  MOTOR. 

The  construction  of  the  motor  will  now  be  explained,  reference  be- 
ing had  to  the  accompanying  drawings.  As  the  motor  is  symmetrical  in 
both  vertical  planes,  the  drawings  of  the  stator  show  it  part  in  full  view 
and  part  in  section. 

The  stator  is  built  up  of  discs  stamped  from  No.  27  transformer 
iron.  These  discs  have  an  internal  diameter  of  5  ins.  and  an 
external  diameter  of  9  ins.  They  should  be  varnished  on 
one  side  with  an  insulating  varnish,  thinned  down  so  as  to 
form  a  thin,  uniform  coating  on  the  surface  of  the  discs.  The  discs  must 
be  dried  before  being  assembled.  Two  disc-shaped  brass  castings  of  the 
same  internal  and  external  diameter  as  the  discs  and  a  thickness  of  3-16 
ins.  serve  as  end  plates.  These  castings  have  four  lugs  on  the  outer 
edge,  strengthened  by  ribs.  Through  these  lugs  the  clamping  rods 
pass. 

A  round  piece  of  sheet  iron,  10  ins.  in  diameter,  should  be  procured, 
from  which  to  make  a  templet.  Find  the  center  of  the  sheet  iron  and 


ELECTRICAL  DESIGNS 


lay  out  a  circle  of  4^3  ins.  radius.  Divide  the  circumference  into  four 
equal  parts  and  centermark  the  division  points.  Drill  the  center  and  the 
division  joints  on  the  circle  with  a  small  drill.  (About  No.  40.)  This 
templet  is  clamped  on  the  brass  castings  so  that  the  holes  on  the  circle 
come  about  over  the  center  of  the  lugs.  With  the  same  small  drill  used 
before  a  hole  should  now  be  drilled  through  the  brass  casting.  These 
holes  arc  then  enlarged  with  a  %-in.  drill.  The  clamping  rods  are  ^4 -in., 
either  Bessemer  or  cold-rolled  steel  rods,  cut  oil  to  6^-ins.  With  a 
34 -in.  standard  die  a  thread  is  cut  on  each  end  of  the  rod  to  a  distance  of 
i%  ins. 

The  disc  may  now  be  assembled.  The  clamping  rods  are  not  strong 
enough  to  properly  compress  the  discs,  and  this  should  be  done  under  a 
drill  press  or  in  a  vice  while  the  nuts  are  tightened  up.  Enough  discs 
must  be  put  on  to  make  the  length  3^  ins.  when  tightened  up.  After 


FIGS.    156  AND   157. — END  AND  SIDE  ELEVATION,    HALF  IN  SECTION. 

the  core  has  been  put  together  it  should  be  chucked  in  a  lathe  and  a 
light  cut  taken  out  of  it  to  make  the  inner  diameter  5  1-64  ins.  Care 
must  be  taken  not  to  make  the  bore  too  large,  as  this  would  much  reduce 
the  efficiency  and  capacity  of  the  motor. 

The  rotor  is  built  up  of  discs  of  the  same  material  as  the  stator,  $£  in. 
internal  diameter  and  5  ins.  external  diameter.  Two  disc-shaped  braes 
castings  serve  as  end  plates.  The  hole  in  the  center  of  these  castings 
should  be  finished  to  ^  ins. 

The  shaft  is  turned  up  from  a  piece  of  cold-rolled  steel  of  xi-in. 


ONE-QUARTER  HORSE-POWER   INDUCTION  MOTOR 


diameter.  The  middle  part  of  the  shaft  is  turned  to  such  a  diameter  that 
the  discs  fit  over  it  and  the  ends  so  as  to  be  a  running  fit  in  |a  ^-in.  hole. 
A  f6-in.  thread  is  cut  on  each  end  of  the  middle  part  of  the  shaft. 

The  discs  for  the  rotor  need  not  be  insulated  but  can  be  built  right 
up  on  the  shaft  and  clamped  by  the  two  hexagon  nuts  shown  in  the 
drawing".  A  hole  is  drilled  into  the  shaft  through  the  end  plates,  as 
shown,  and  a  steel  rod  or  round  spike  is  driven  into  the  hole  and  saved 
off.  The  rotor  is  now  put  into  a  milling  machine,  and  with  a  J/&-in. 


FIG.    158. — FACE  AND  EDGE  VIEWS   OF  AN  END  DISC. 

milling  cutter,  15  slots  are  cut  3-16  in.  deep.  The  No.  8  copper  wires 
have  to  be  driven  into  these  slots.  They  are  soldered  at  the  ends  to  the 
brass  end  plates.,  the  solder  being  applied  liberally  and  made  to  fill  up  all 
around  the  wire.  The  rotor  should  now  be  put  in  a  lathe  and  turned 
down  to  a  diameter,  of  4  63-64  ins.  Care  must  also  be  exercised  here  to 
avoid  taking  too  large  a  cut.  While  the  rotor  is  in  the  lathe,  the  nuts 
on  the  shaft  are  turned  to  y%  in.  in  length. 

We  now  take  the  bearing  supports,  and  by  means  of  our  templet 
drill  the  holes  for  the  clamping  rods.     In  fastening  the  templet  to  the 


ELECTRICAL  DESIGNS 

castings  a  center  should  be  made  to  coincide.  In  one  of  the  castings 
two  y&-\n.  holes  for  the  binding  screws  are  drilled  through  the 
center  of  the  bosses  provided  for  this  purpose.  A  piece  of  soft  wood  is 
cut  that  will  fit  into  the  oil  chamber.  A  3^ -in.  hole  is  drilled  through 
tills  piece  of  wood,  through  which  the  shaft  may  pass. 

The  rotor  is  now  wrapped  with  paper  until  it  fits  tightly  in  the 
stator.  It  is  put  into  place  and  the  bearing  supports  are  slipped  on  with 
the  wood  in  place  in  the  oil  chambers.  The  bearing  supports  are  fast- 
ened down  by  means  of  nuts  on  the  clamping  rods.  Some  babbitt  metal 
should  now  be  melted  in  a  ladle,  the  motor  set  on  end  and  the  outer  parts 
<of  the  bearings  filled  with  babbitt.  The  bearing  supports  should  now  be 
snarked  so  that  they  can  be  put  on  the  same  way  again  after  they  have 
teen  taken  off.  Take  off  the  bearing  supports,  reverse  the  shaft  through 
them  and  fill  up  the  remaining  end  with  babbitt.  The  ends  of  the 
Jbabbitt  lining  should  now  be  trimmed  up,  the  bearings  put  back  on  the 
core  and  a  3/2 -in.  reamer  run  through  them. 

We  now  take  the  stator  core  and  put  it  in  a  shaper  to  cut  the  slots. 
The  slots  are  182  mils  wide  and  426  mils  deep,  and  there  are  32  of  them 


FIG.    159. — DETAILS    OF    ROTOR. 

equally  spaced  around  the  inner  circumference  of  the  stator  core.  After 
Ihe  slots  have  been  cut,  all  the  sharp  edges  are  rounded  off  with  a  file 
and  the  core  is  cleaned  of  all  iron  dust  and  grease.  The  insulation  is 
:next  put  on.  Shellac  dissolved  in  alcohol  is  the  best  insulating  substance 
to  stick  the  insulating  material  to  the  core.  Two  thicknesses  of  press- 
board,  15  mils  thick  should  be  used  in  the  slots,  the  troughs  being  made 
about  y&  in.  longer  than  the  core.  The  ends  and  outside  of  the  core  are 
insulated  in  the  same  manner  as  ordinary  direct-current  armatures,  and 
it  will  not  be  necessary  to  describe  this  specially.  Any  one  not  familiar 
with  the  method  of  insulating  armature  cores  may  refer  to  the  descrip- 
tions of  small  motors  in  Chapters  I  to  IX. 

The  wire  is  wound  in  the  slots  two  wide  and  six  deep,  while  on  the 
cnitside  of  the  core  it  is  wound  only  two  deep.    The  coils  are  connected 


ONE-QUARTER   HORSE-POWER  INDUCTION  MOTOR  139 

as  shown  in  the  diagram.  In  the  first  eight  coils  the  ending  of  one  coil 
is  connected  to  the  beginning  of  the  next.  The  ending  of  the  eighth, 
coil  is  connected  to  the  ending  of  the  ninth,  the  beginning  of  the  ninth  to 
the  end  of  the  tenth  and  so  on  until  the  sixteenth.  The  beginning  of  the 
sixteenth  is  connected  to  the  beginning  of  the  seventeenth,  the  end  of  the 
seventeenth  to  the  beginning  of  the  eighteenth,  and  so  on  to  the  twenty^ 
fourth,  where  the  connections  are  changed  again.  The  beginning  and 
ending  of  the  whole  winding  are  brought  out  to  the  binding  posts.  These 
consist  of  54 'm-  round-head  machine  screws,  il/\.  inches  long,  passing" 
through  the  bearing  support,  being  insulated  from  it  by  fiber  washers 
and  bushings.  A  tube  of  sheet  iron  is  made,  10  ins.  diameter  and  5  ins~ 
long  which  will  just  fit  over  the  circular  offset  on  the  bearing  supports 
and  serve  to  protect  the  windings  of  the  stator.  This  completes  every 
part  of  our  motor,  and  it  may  now  be  assembled. 

The  motor  is  not  self-starting,  and  has  to  be  brought  up  to  speed 
by  some  external  means.  If  the  bearings  are  well  aligned,  as  they" 
should  be,  a  vigorous  start  by  hand  on  the  pulley  will  be  sufficient  to 
make  the  motor  pick  up.  The  motor  should  be  started  immediately  the 
current  is  turned  on,  and  for  this  reason  a  switch  should  be  placed  con- 
venient to  the  motor. 


CHAPTER  XVI. 


SIMPLE   TRANSFORMER   IN   FOUR  SIZKS. 


The  transformers  here  described  can  be  built  by  any  amateur  with- 
out the  use  of  machine  tools;  some  form  of  winding  machine  being-  the 
most  important  piece  of  constructive  apparatus.  In  order  to  eliminate 
the  bugbear  of  stampings,  the  core  is  made  of  an  unusual  type,  involving 
the  use  of  simple  rectangular  strips  of  transformer  sheet  iron,  No.  27 


FIG.    l6o. — SECTIONAL   PLAN. 


FIG.  l6l. — WINDING  CORE  BLOCK. 


gauge,  all  of  one  size.  The  coils  surround  the  core  exactly  like  the 
windings  of  an  induction  coil,  and  the  core  sheets  are  bent  back  around 
the  outside  of  the  coils  and  lapped,  as  indicated  in  Fig.  160,  which  is  a 
horizontal  section  through  the  center  of  the  transformer,  with  most  of 
the  core  sheets  omitted  beyond  the  ends  of  the  winding.  All  four  sizes 
for  which  data  are  given  are  of  identical  construction,  the  only  difference 
being  dimensional. 

The  first  step  is  to  make  two  wooden  core  blocks,  like  Fig.  161 ;  one 
on  which  to  mount  the  primary  bobbin  and  one  for  the  secondary  bobbin. 


SIMPLE  TRANSFORMER  IN  FOUR  SIZES 


141 


One  head  may  be  put  on  permanently,  but  the  other  must  be  removable. 
The  dimensions  are  given  in  Table  I.  A  spindle  of  24-in.  round  iron 
should  be  put  through  the  center  of  the  core  lengthwise,  and  if  a  lathe  is 
not  available  for  winding  purposes,  one  end  of  the  spindle  may  be  bent 
into  a  crank  and  the  whole  structure  mounted  between  two  simple  up- 
right posts,  to  form  a  winding  machine. 

The  next  step  is  to  make  the  retaining  bobbin  for  the  primary  wind- 
ing. Take  a  sheet  of  heavy  fuller  board,  of  the  size  specified  in  the  table 
Df  dimensions,  and  cut  four  slits  in  each  long  edge,  as  shown  in  Fig.  162 
at  a  and  b ;  these  slits  are  %  in.  long  in  all  cases.  Bend  the  sheet  into  the 
shape  shown  at  Fig.  163,  forming  a  sort  of  box  with  open  ends,  and  slip 


A                             c/ 

t 

f 

i 

/\,   !      ,  ,  ,.,-.,   „,  „ 

c/ 
c 

mHiii      j 

7 

c                                                       c 
i                          > 

b                                             b 

FIG.    163, — BOBBIN   CORE, 


FIG.    l62. — BOBBIN   CORE   SHEET. 


FIG.    164. — COLLAR. 


over  the  outside  six  rectangular  collars  of  vulcanized  fibre,  like  Fig.  164; 
these  are  1-16  in.  thick.  Bend  the  flaps,  c,  Fig.  163,  outwardly  at  right 
angles  with  the  wall  of  the  "box,"  so  that  when  the  collars  are  finally  ad- 
justed into  place  along  the  outside,  each  end  one  will  be  held  on  by  four 
flaps,  as  indicated  in  Fig.  165.  The  surfaces  of  the  flaps  may  be  coated 
with  thick  shellac  varnish  in  order  to  keep  them  in  place  against  the  faces 
of  the  end  collars. 

Next,  mount  the  complete  bobbin,  Fig.  165,  on  its  wooden  core 
block,  Fig.  161,  and  prepare  it  for  the  primary  winding.    The  partitions 


I42 


ELECTRICAL   DESIGNS 


or  collars  must  be  adjusted  at  equal  distances  apart,  as  in  Fig.  165,  and 
the  spacing  maintained  temporarily  by  means  of  wooden  blocks.  Before 
applying  the  winding,  the  seam  where  the  edges  of  the  fuller  board  meet 
must  be  covered  by  a  strip  of  the  same  material  laid  clear  across  the  side 
of  the  box  in  each  of  the  compartments  and  secured  in  place  by  varnish. 
Then  the  coils  may  be  wound  on,  care  being  taken  to  observe  rigidly  the 
prescription  of  Table  II  as  to  number  of  turns  per  section.  The  starting 
and  finishing  ends  of  each  coil  or  section  must  be  of  heavier  wire  than 
that  of  the  coil  itself,  and  rubber-covered  with  an  outer  braid;  No.  18 
wire  is  a  good  size  for  the  two  smaller  sizes  of  transformer,  and  No.  14 
for  the  two  larger  ones. 

After  winding  and  securing  the  ends  with  heavy  linen  thread,  tag  the 
ends,  marking  the  inner  or  starting  ends,  "B,"  and  the  outer  or  final  ends, 
"F."  Then  tape  each  section  thoroughly  and  lead  the  terminals  of  the 
various  sections  lengthwise  along  the  outside  of  the  whole  winding  to  one 


FIG.    165. — COMPLETE  BOBBIN. 


FIG.    166. — CORE-CLAMPING   DOGS. 


end,  securing  these  terminal  wires  to  the  surface  of  the  structure  by 
means  of  a  few  extra  turns  of  tape.  All  of  the  terminals  should  project 
from  one  end  of  the  complete  winding,  and  they  should  be  laid  side  by 
side  in  regular  order. 

The  secondary  bobbin  or  box  is  made  in  exactly  the  same  way  as  the 
primary,  but  has  only  five  collars  instead  of  six,  and  is  larger  in  size,  as 
Table  I  shows.  After  winding  the  secondary,  tape  it  on  the  outside  and 
tag  the  ends,  like  the  primary;  varnish  both  heavily  with  either  P.  &  B. 
or  shellac  varnish,  and  set  them  aside  to  dry. 

Next  mount  between  two  pairs  of  wooden  dogs  the  requisite  number 
of  core  plates  to  make  the  proper  thickness,  as  indicated  in  Figs.  166  and 
167,  drawing  the  dogs  up  snug,  and  wrap  the  core  plates  tightly  with 
three  layers  of  plain  linen  tape  along  the  portion  between  the  dogs. 


SIMPLE    TRANSFORMER  IN  FOUR  SIZES 

'::'i:>""!l!l 


FIG.    167.— CORE-CLAMPING  DOGS. 


FIG.    168. — JOINT  CLAMP. 


FIG.    l6g. — TABLET  BOARD  DIAGRAM. 


144  ELECTRICAL  DESIGNS 

Varnish  the  outer  layer  of  tape  lightly;  remove  the  pair  of  dogs,  and  slip 
on  the  primary  and  secondary  windings  separately.  The  windings  should 
be  so  disposed  that  the  sides  out  of  which  the  terminals  lead  are  on  top. 
When  the  windings  are  in  place,  put  back  the  pair  of  dogs  previously  re- 
moved and  bend  the  core  plates  around  the  ends,  lapping  them  as  indi- 
cated in  Fig.  160;  one-half  of  the  plates  should  be  carried  around  one 
side  and  one-half  around  the  other.  The  lapped  joints  must  be  tightly 
clamped  together  by  means  of  two  wooden  strips  iy2  ins.  wide  and  2  ins. 
thick,  drawn  together  with  ^-in.  iron  bolts,  as  shown  by  Fig.  168. 

To  the  upper  ends  of  the  four  dogs  which  hold  the  core,  screw  a 
tablet  board  to  which  the  terminals  of  the  windings  are  led  and  from 
which  the  main  transformer  connections  are  made.  The  tablet  board  is 
most  easily  made  of  "soapstone  slate."  which  is  merely  a  very  soft  grade 
of  light  gray  slate.  Wood  or  fiber  will  not  do  and  hard  rubber  would  be 
expensive.  On  it  mount  eight  single-pole  double-throw  "baby"  knife- 
blade  switches,  two  primary  terminal  blocks,  eight  heavy  binding  posts, 
and  two  secondary  terminal  blocks,  as  indicated  in  Fig.  169.  The  sketch 
also  shows  how  the  coils  are  connected  to  the  switches  and  binding  posts. 

The  transformers  are  all  designed  for  1,000  volts  primary  e.m.f.,  with 
all  of  the  primary  coils  in  series,  and  100  volts  secondary  e.m.f.  with  all  of 
the  secondary  coils  in  "series.  The  primary  winding  is  divided  into  five 
equal  sections  of  200  volts  each,  so  that  it  can  be  grouped  for  200,  400, 
600,  800  or  1,000  volts.  In  grouping  for  400  and  800  volts  one  section 
must  be  left  open,  and  in  grouping  for  600  volts  two  sections  must  be  left 
disconnected  from  the  main  primary  terminals.  The  original  object  of 
the  writer  in  dividing  the  primary  into  2OO-volt  sections  was  to  permit  the 
transformer  to  be  supplied  from  either  an  ordinary  i,ooo-volt  primary 
circuit  or  a  2OO-volt  motor  circuit.  The  reader  will  readily  understand 
that  this  arrangement  is  not  compulsory ;  the  primary  may  be  wound  in 
a  single  coil  without  any  partitions,  if  desired,  although  it  will  be  found 
more  reliable  if  two  or  three  partitions  be  used  to  reduce  the  voltage  per 
section. 

The  secondary  winding  as  designed  is  divided  into  two  sections  of 
about  1 8  volts  each  and  two  sections  of  about  32  volts  each. 
No  switches  are  used  for  making  the  various  combinations 
because  too  much  space  and  complication  would  be  required. 
The  connections  are  to  be  made  between  the  various  binding  posts  by 
means  of  short  lengths  of  heavy  iron — No.  6  or  No.  8  gauge!  In  order  to 
avoid  excessive  ohmic  loss  the  wire  should  fit  the  hole  in  the  binding 
post  snugly.  All  of  the  binding  posts  should  have  two  holes  and  binding 
screws  each. 


SIMPLE   TRANSFORMER  IN  FOUR  SIZES 


With  primary  switches  all  thrown  inward,  the  primary  sections  are 
in  series  for  1,000  volts  between  the  terminal  posts.  With  all  of  them 
thrown  outward  the  primaries  are  in  multiple  for  200  volts.  Other  com- 
binations may  be  easily  traced  out.  At  the  secondary  end  several  com- 


TABLE   I.  —  MECHANICAL    DIMENSIONS. 

Size  of  transformer   watts  .    ...        

200 

20  ins. 

I# 

1% 

rA 

47A 
43A 

I* 
tf 

12^ 

47/s 

4/8 

3l/& 

l/2 
3J/2 

I 
xtf 
i# 

2^ 

500 

22  ins. 

2^ 
2^ 

10^ 

5* 
4* 

1 

15^ 
4# 

4ti 
37/* 
% 

4X 

i 

2 
Itf 

3 

750 
30^  ins. 

2^ 
2^ 

»£ 

5TJr 

2^J 

*6 

1^/2 

5* 

5T> 
41 

4l/2 

I 

2 

1% 

3 

IOOO 

31%  ins. 
3 
3 

!2# 

5/8 

4^ 

^ 
l^ 

47/s 
4*A 

H 

4% 
I 

2^ 
2 

3K 

Length  of  core  plates   

Width          "         " 

Thickness  of  compressed  core   d 

Primary  Bobbin. 
I  ength  of  sheet   C  

Width             "       L  

Length  of  finished  bobbin 

Width  of  one  side  S 

Depth  of  flanges                                 .  .    .  . 

Secondary  Bobbin. 
Length  of  sheet,  C  

Width             "       L  

Length  of  finished  bobbin 

"Width  of  one  side    S                             

Depth  of  flanges                    

Core  Dogs,  D 
Depth  of  core  clamp   e 

Thickness  parallel  with  the  core       

Thickness  parallel  with  the  bolts  

Height   foot  to  core,  f  

Length  of  foot  g 

TABLE   II. — ELECTRICAL   AND   MAGNETIC   DATA. 

At  1000  volts  primary  and  100  volts  secondary;  133  cycles. 


Output  of  transformer  watts  . 

200 

COO 

IOOO 

Primary  Winding. 

No.  26 

No    22 

No  20 

No  10 

Turns  per  section 

640 

2C.O 

220 

3,200 

I,  COO 

I,25O 

I,IOO 

Depth  of  winding  layers  

20 

12 

II 

128 

3O 

18 

C8R  loss,  full  load  

10  1A 

.fix 

Secondary  Winding. 
Size  of  wire    B   &S  

No  16 

No  ii 

No  10 

No  9 

Turns  per  small  section  

27 

23 

20 

"     large      "      

103 

48 

4O 

35 

320 

ICQ 

126 

no 

5 

4 

3 

Resistance   hot             

2 

O.2 

o.  175 

C8R  loss,  full  load  

8 

oy 

Hff 

Losses,  Full  Load. 

16 

21.5 

30^ 

Hysteresis.  

_ 

8 

IO.7 

jT/ 

i 

0.8 

i 

7» 

Total  losses  

28 

25 

33 

40 

QS53X 

06  Y 

° 

0 

i^O  ELECTRICAL   DESIGNS 

binations  are  obtainable,  as  shown  by  Fig.  170;  the  most  serviceable  will 
doubtless  be  found  to  be  the  one  on  the  left  of  the  sketch. 

The  tablet  board  must  be  kept  enclosed  by  a  box  cover  when  the 
transformer  is  in  use.  The  fuse  blocks  and  master  switches  should  be  lo- 
cated at  a  little  distance  from  the  transformer,  and  the  cover  should  never 
be  removed  except  when  the  primary  switch  is  open. 


CHAPTER  XVII. 


CONSTRUCTION  OF  A  REACTIVE  COIL. 


It  is  well  known,  of  course,  that  reactive  or  "choking"  coils  are  used 
in  alternating-current  work  instead  of  ordinary  resistance  coils  for  the 
purpose  of  reducing  the  e.m.f.  in  a  portion  of  a  circuit  because  they  are 
much  less  wasteful  than  resistance  coils  or  rheostats.  As  the  uses  of  re- 
active coils  are  so  diverse  no  one  design  can  be  given  which  will  fit  all 
cases ;  hence  only  one  form  will  be  described  here  in  detail  and  rules  will 
be  given  by  means  of  which  anyone  can  modify  the  design  to  fit  any  case. 

The  reactive  coil  here  described  is  designed  for  use  in  series  with 
one,  two  or  three  open  arcs  on  a  constant-potential  circuit  of  100  or  no 


FIG.    171. — THE   CORE. 


FIG.    172. — TEMPLATE. 


volts,  or  with  one  lamp  on  a  50-55  volt  circuit,  or  with  three  enclosed  arc 
lamps  fed  from  a  200  volt  circuit.  Its  dimensions  and  windings  are  based 
upon  a  magnetic  density  of  30,000  lines  of  force  per  square  inch  in  the 
core  and  it  may  be  adjusted  to  pass  any  current  from  y2  ampere  to  15 
amperes.  The  apparatus  is  of  the  adjustable  core  type,  and  if  desired,  the 
winding  can  be  tapped  at  various  points  and  the  coil  can  be  used  as  an 
auto-transformer. 

The  core  consists  of  rectangular  plates  of  No.  27  transformer  iron, 
14  ins.  long  and  2  ins.  wide,  bent  into  the  form  shown  by  Fig.  171.  The 
thickness  of  the  core  must  be  2  ins.  The  easiest  way  to  assemble  it  is  to 
make  an  iron  template  like  Fig.  172,  of  J/2-inch  strap  iron  and  bend  the 
successive  plates  of  the  core  into  shape  over  the  template,  one  at  a  time, 


148 


ELECTRICAL  DESIGNS 


leaving  them  in  position  as  they  are  bent.  Clamp  the  strips  to  the  tem- 
plate as  shown  in  Fig.  173  and  bend  the  ends  down  without  resorting  to 
any  hammering  whatever.  The  core  must  measure  2  ins.  in  thickness 
when  clamped  tightly.  As  each  strip  is  bent  down  into  shape  its  ends 
should  be  squared  off  with  a  pair  of  tinners'  snips,  so  that  when  all  are 


FIG.    173. — CORE  STRIPS  ON  TEMPLATE. 

bent  the  ends  will  all  be  flush,  forming  a  laminated  pole-face  at  each  end 
of  the  core,  as  indicated  in  Fig.  171. 

When  the  last  strip  is  in  place,  remove  the  template,  replace  the 
clamps  near  the  corners  of  the  core  and  bind  the  core  strips  tightly  to- 
gether with  heavy  cord  (at  least  1-16  in.  in  diameter),  winding  a  full  layer 
from  bend  to  bend,  and  pulling  each  turn  just  as  tight  as  the  cord  will 


FIG.    174. — WOODEN  CLAMPING  BLOCKS. 

stand  it.  The  best  way  is  to  take  a  couple  of  turns  at  one  corner  and  tie 
the  cord ;  then  loosen  the  clamp  and  move  it  an  inch  away,  setting  it  up 
tight  again ;  wind  the  cord  over  this  inch  of  space  and  move  the  clamp 
another  inch,  continuing  this  procedure  until  the  whole  core  is  covered 
between  the  two  bends.  After  binding  the  core  with  cord  in  this  manner, 
cover  it  with  two  layers  of  insulating  tape,  carrying  the  tape  around  the 
bend  and  out  almost  to  the  end  of  the  right-angle  poles.  Then  wind  on 
that  portion  of  the  core  between  the  bends  200  turns  of  No.  8  double  cot- 


CONSTRUCTION  OF  A  REACTIVE  COIL 


149 


ton-covered  magnet  wire  in  four  layers.  When  this  is  done  secure  the 
ends  of  the  core  between  three  clamping  blocks  mounted  on  a  base 
board,  as  shown  in  Fig.  174. 

The  yoke  which  completes  the  magnetic  circuit  is  somewhat  similar 
in  form  to  the  core  just  described,  as  Fig.  175  indicates,  but  the  right- 
angle  projections  at  each  end  of  the  yoke  are  much  shorter  than  those  of 
the  main  core.  The  exact  length  of  these  projections  is  immaterial  ex- 
cept that  it  should  be  not  less  than  an  inch  and  not  more  than  two.  The 
yoke  will  preferably  be  built  up  in  the  same  way  as  above  described  in 
connection  with  the  main  core,  and  after  it  is  bound  together  with  twine 
it  should  be  mounted  in  the  clamp  shown  in  Fig.  176  in  such  a  way  that 
the  center  of  the  spindle  projecting  from  one  side  of  the  clamp  will  coin- 
cide with  the  center  of  the  yoke  structure.  The  clamp  jaw  should  be 
made  a  snug  fit  for  the  yoke  so  that  the  bolts  will  not  need  to  be  drawn 
up  very  tightly.  The  bolts  must  be  insulated  from  the  metal  of  the  clamp 


Nut 


FIG.    175. — THE  YOKE. 


FIG.    176. — CLAMP  FOR  YOKE. 


by  bushings  of  either  hard  fiber  or  rubber  in  order  to  avoid  forming  a 
closed  circuit  around  the  yoke,  which  would  result  in  a  heavy  flow  of 
current  through  the  clamp  and  bolts. 

The  spindle  is  seated  in  a  bushed  hole  through  the  center  of  the 
long  clamping  block  shown  in  Fig.  174,  so  that  the  ends  of  the  yoke  can 
be  brought  into  alignment  with  the  pole-faces  of  the  main  core.  A 
round-nose  set  screw  through  the  upper  edge  of  the  clamping  block  will 
serve  to  hold  the  spindle  in  any  position  to  which  it  may  be  adjusted. 
Two  adjustments  are  available,  one  in  a  rotary  direction  about  the  cen- 
ter of  the  spindle,  and  the  other  in  a  straight  line  toward  and  away  from 
the  pole-faces  of  the  main  core.  This  latter  adjustment  is  preferably  made 
by  means  of  a  thin  nut  fitted  to  a  fine  screw  thread  on  the  spindle,  as  in- 
dicated in  Fig.  176,  the  set-screw  in  the  clamping  block  being  used  mere- 
ly to  secure  the  core  in  any  position  to  which  it  may  be  adjusted.  To  use 
the  apparatus  as  a  choking  coil,  connect  the  winding  in  series  with  the 
lamp  or  lamps,  adjust  the  regulating  nut  on  the  spindle  so  as  to  secure 
the  length  of  air  gap  specified  in  the  accompanying  table  and  secure  finer 
gradations  by  twisting  the  yoke  into  or  out  of  alignment  with  the  pole- 


150 


ELECTRICAL  DESIGNS 


faces  of  the  main  core  until  the  exact  choking  effect  is  secured,  when  the 
set-screw  may  be  used  to  hold  the  yoke  in  that  position.  The  length  of 
air-gap  given  in  the  body  of  the  tables  refers  to  each  of  the  two  gaps,  not 
the  sum  of  the  two. 

If  it  should  be  desired  to  use  the  apparatus  as  an  auto-transformer 
the  yoke  should  be  brought  into  accurate  alignment  with  the  pole  faces, 
pushed  up  solidly  against  them  and  held  in  this  position  permanently  by 
means  of  the  set-screw.  Used  in  this  manner,  the  winding  will  have  to 


LENGTH   OF  AIR   GAP  ;    OPEN  ARC  LAMPS. 


I  Lamp; 

Lamps  on  ic>4-volt  circuit. 

Ampere 

52-volt 

circuit. 

I 

2 

3 

4 

& 

A 

A 

% 

6.6 

X 

A 

TV 

y* 

7-5 

*J 

A 

H7T 

H 

10 

H 

A 

A 

i# 

12 

A 

A 

A 

*X 

14 

% 

7 

7T 

A 

i# 

LENGTH    OF  AIR    GAP  J    ENCLOSED   ARCS. 


i  Lamp; 

Lamps  on  2Oo-volt  circuit. 

Ampere. 

loo-volt 

circuit. 

i 

2 

3 

4 

5 

I 

A 

X 

6.6 

7-5 

1 

¥ 

A 

.  &  --  • 

10 

A 

A 

TV 

13 

'X 

TV 

A 

I 

be  tapped.  Each  turn  of  the  winding  will  represent  1-200  of  the  e.m.f.  of 
the  circuit,  so  that  to  operate  a  single  33-volt  lamp  on  a  5O-volt  circuit 
one  terminal  of  the  lamp  must  be  tapped  into  the  winding  of  -the  auto- 
transformer  133  turns  distant  from  the  other  terminal,  or  preferably  at 
2-3  the  distance  from  one  end,  as  indicated  in  Fig.  177.  Fig.  178  indi- 
cates the  arrangement  of  the  taps  for  three  enclosed  arc  lamps  on  a  200- 
volt  circuit.  Taps  would  be  taken  out  at  exactly  the  same  points  to  sup- 
ply three  open  arc  lamps  on  a  loo-volt  circuit. 

Should  the  reader  desire  to  construct  a  reactive  coil  to  suit  any  other 
conditions,  the  following  simple  formulas  will  give  the  required  dimen- 


CONSTRUCTION  OF  A  REACTIVE  COIL 


15* 


sions  and  winding.  For  work  on  a  133-cycle  circuit  the  choking  effect  in 
volts  for  a  coil  built  for  the  average  conditions  of  practice  is  given  by  the 
formula : 

In  this  formula  A  is  the  area  of  the  core  cross-section  and  T  is  the  num- 
ber of  turns  of  wire.  In  order  to  make  this  formula  hold  good  the  num- 
ber of  turns  of  wire  must  agree  with  the  formula : 

34  L  -  T  (2) 


C 


=  T 


In  this  formula  L  is  the  length  of  the  magnetic  circuit  within  the 


FIG.   177.— CONNECTIONS  AS  AUTO-TRANS- 
FORMER 1   ONE  LAMP. 


FIG.   178. — THREE  ENCLOSED  ARCS   ON  2OO- 
VOLT   CIRCUIT. 


iron,  including  the  yoke,  'and  C  is  the  current  in  the  winding.    For  60- 
cycl,e  circuits 

o.n  XA  X  T  =  E      ......       (3) 

in  which  the  number  of  turns  must  agree  with  the  formula  : 


_,   -  =  ^    •:,•-•   .....     (4) 

The  two  joints  between  the  yoke  and  the  main  core  when  they  are  in 
actual  contact  are  equivalent  to  about  36  ins.  of  iron  under  the  conditions 
assumed  in  formulas  (i)  and  (2),  and  to  about  52  inches  under  the  con- 
ditions upon  which  formulas  (3)  and  ('4)  are  based  ;  therefore,  in  figuring 


152  ELECTRICAL  DESIGNS 

the  closed  circuit  either  36  or  52,  as  the  case  may  be,  must  be  added  to 
the  actual  length  of  the  magnetic  circuit  in  the  iron. 

The  size  of  the  wire  to  be  used  on  a  coil  may  be  ascertained  by  al- 
lowing 1,000  circular  mils  of  cross  section  per  ampere  of  current.  In 
designing  a  reactive  coil  the  dimensions  for  maximum  output  should  be 
calculated,  first  assuming  the  yoke  in  contact  with  the  main  core,  because 
anything  below  the  maximum  effect  can  be  obtained  by  adjusting  the  dis- 
tance between  the  yoke  and  the  main  core. 

As  an  example,  suppose  it  were  desired  to  build  a  coil  to  regulate  the 
impressed  e.m.f.  of  a  circuit  of  6.6-ampere  lamps  fed  from  a  2OO-volt 
transformer,  on  a  133-cycle  circuit,  the  range  of  regulation  or  choking 
effect  being  from  25  volts  to  150.  Transposing  formula  (i)  into: 


o.i8x  r" 

and  assuming  temporarily  50  as  the  number  of  turns,  the  area  of  the  core 
will  be  for  the  maximum  reactive  effect  of  150  volts,  2p  sq.  ins.  Making 
the  core  4x5  ins.  will  give  this  area. 

Now  transposing  formula  (2)  to  read : 


34 

and  substituting  for  C  and  T  the  values  of  6.6  and  50  respectively,  we  find 
that  the  length  of  the  magnetic  circuit  (if  it  were  all  iron)  would  need  to 
be  97  ins.,  which,  of  course,  would  be  a  ridiculous  dimension.  It  must 
be  remembered,  however,  that  an  air-gap  is  equal  to  practically  1,  800 
times  its  length  of  good  sheet  iron  under  the  conditions  assumed  in 
formulas  (i)  and  (2)  ;  therefore  it  is  only  necessary  to  make  the  iron  core 
long  enough  to  accommodate  the  winding  and  then  insert  an  air-gap  of 
such  a  length  as  to  bring  the  total  reluctance  equal  to  that  of  97  ins.  of 
iron. 

No.  12  wire  will  be  large  enough  to  carry  the  current,  and  the  diam- 
eter of  this  over  the  insulation  is  0.092  in.  ;  50  turns  side  by  side,  therefore, 
will  make  a  coil  4^  ins.  long,  which  is  not  excessive.  The  total  length  of 
the  iron  part  of  the  magnetic  circuit  may  be  made  about  20  ins.,  so  that 
the  two  air-gaps  must  be  made  equivalent  to  97  —  20  =  77  ins.  of  iron. 
As  each  inch  of  air-gap  is  equal  to  1,800  ins.  of  iron,  the  total  length  of 
air-gap  required  will  be  : 


.      _ 

so  that  each  air-gap  will  be  about  7-64  inch  long  in  order  to  bring  the 
choking  effect  of  the  coil  down  to  150  volts  with  6.6  amperes  flowing 
through  the  winding. 


CONSTRUCTION  OF  A   REACTIVE  COIL  153 

The  foregoing-  formulas  and  instructions  are  based  on  a  magnetic 
density  of  30,000  lines  per  square  inch  in  the  core  of  a  coil  to  operate  on. 
a  i33-cycle  circuit,  and  52,000  lines  per  square  inch  in  the  core  of  a  coil 
to  work  on  a  6o-cycle  circuit.  While  the  density  can  be  carried  somewhat 
higher  than  this  and  the  size  of  the  core  correspondingly  decreased,  more 
satisfactory  results  will  usually  be  obtained  by  employing  the  densities 
here  given,  because  with  higher  densities  the  hysteresis  loss  in  the  core 
will  cause  it  to  overheat  and  jeopardize  the  coil.  The  two  densities  above 
specified  give  the  same  core-loss,  to  wit :  o  .68  watt  per  cubic  inch  of 
core  iron> 


CHAPTER  XVIII. 


THE  CONSTRUCTION  AND  CALCULATION  OF  RHEOSTATS. 


The  resistance  material  of  rheostats  for  the  regulation  of  current  or 
potential  in  electrical  circuits  may  be  metallic  wire,  carbon  or  graphite, 
or  acidulated  water.  In  the  present  article  rheostats  in  which  the  first 
named  material  is  employed  will  only  be  considered. 

The  different  conductor  materials  used  in  the  construction  of  com- 
mercial rheostats  are  iron,  German  silver  and  copper.  Each  of  these 
materials  has  advantages  in  particular  cases.  The  advantages  of  iron  are 
cheapness  and  the  ability  to  withstand  high  temperatures.  German  silver 
lias  a  high  resistivity  or  specific  resistance  and  a  low  temperature  co-effi- 
cient. Copper  is  only  used  where  large  currents  have  to  be  carried,  as, 
for  instance,  in  electro-plating  work,  where  one  dynamo  supplies  several 
tanks  requiring  different  voltages,  and  regulation  is  effected  by  inserting 
resistance  into  the  circuits  requiring  the  lower  pressure.  In  this  case, 
copper,  by  virtue  of  its  higher  conductivity,  makes  it  possible  to  use 
smaller  conductors,  thus  facilitating  the  construction  of  the  rheostat. 

The  table  on  page  155  gives  the  carrying  capacity  of  tinned  iron 
wire  under  different  conditions.  The  last  column  gives  the  length  of 
wire  having  a  resistance  of  one  ohm. 

In  designing  motor- starting  rheostats,  the  values  given  under  the 
heading  "Safe  current  for  one  minute  "  should  be  used,  while  the  carry- 
ing capacities  given  in  the  other  two  columns  apply  to  dynamo  field  rheo- 
stats, motor  regulators  and  such  other  rheostats  as  have  to  carry  current 
continuously.  No  definite  resistivity  and  carrying  capacity  can  be 
assigned  to  German  silver,  as  it  is  an  alloy,  and  different  makers  use 
different  proportions  of  the  elements.  In  the  tables  given  by  Matthiesen 
the  resistivity  of  German  silver  is  given  as  2.2  times  that  of  iron.  For 
the  same  rise  of  temperature  a  German  silver  wire  would,  therefore,  carry 
about  two-thirds  the  current  of  an  iron  wire  of  the  same  size.  Most 
commercial  German  silver  has,  however,  a  specific  resistance  higher  than 
that  indicated  by  the  above  ratio. 


CONSTRUCTION  AND  CALCULATION  OF  RHEOSTATS 


155 


The  wires  of  rheostats  are  mounted  in  a  number  of  different  ways. 
They  may  be  embedded  in  enamel  or  some  other  refractory  insulating 
material;  they  may  be  wound  on  a  plate  or  slate;  they  may  be  wound 
on  a  framework  of  iron  rods  insulated  with  asbestos,  or  on 


Size  of  Wire, 
B.  &  S. 

Safe  Current  in 
Wood  Frame. 

Safe  Current  in 
Iron  Frame. 

Safe  Current  for 
One  Minute. 

Feet  per 
Ohm. 

S 

17.4 

20.3 

43-6 

250 

9 

14.6 

17.1 

36.6 

173 

10 

12.3 

14-3 

30.8 

137 

ii 

10.3 

12.0 

25.8 

108 

12 

8.7 

10.  1 

2L7 

86.4 

13 

7-3 

8.5 

18.3 

68.5 

14 

6.1 

7-i 

15-3 

54.3 

15 

5-i 

6.0 

I2.Q 

43-r 

16 

4-3 

5-0 

10.8 

34-  r 

17 

3-6 

4.2 

9-1 

27.1 

18 

3-00 

3-5 

7-6 

24.3 

19 

2.52 

2.9 

6.3 

16.5 

20 

2.17 

2-5 

5-4 

13-5 

21 

1.82 

2.1 

4.5 

10.7 

22 

1-53 

1.77 

3-8 

8-49 

23 

1.28 

1.49 

3  2 

6.73 

24 

i.  08 

1.  20 

2-3 

5134 

insulated  metallic  spools  with  layers  of  asbestos  between  the 
layers  of  wire.  Finally,  the  wire  may  be  wound  into  coils  which  are 
stretched  between  insulators  on  an  iron  frame  or  in  a  frame  of  insulating 
material.  When  the  wires  are  embedded  in  enamel,  they  are  placed  on  the 
surface  of  and  in  close  proximity  to  a  cast-iron  base  plate  which  assists  in, 
radiating  the  heat.  Slate  is  also  quite  a  good  conductor  of  heat,  and 
plates  of  slate  are  often  used  for  smaller  rheostats.  Spool-wound  coils 
of  wire  sometimes  present  an  advantage  where  the  rheostat  is  only  used 
for  a  short  period  at  a  time,  as,  for  instance,  in  motor-starting  rheostats, 
as  this  method  of  winding  permits  of  getting  a  large  amount  of  wire  into 
a  small  space,  and  the  capacity  of  the  rheostat  under  such  conditions  de- 
pends more  on  its  capacity  for  taking  up  heat  than  on  the  radiation.  When, 
spiral  coils  are  employed,  they  are  generally  placed  in  a  case  with  open- 
ings to  facilitate  the  circulation  of  air. 

The  diameter  to  which  spiral  coils  of  wire  are  wound  varies  with  the 
size  of  the  wire.  If  for  a  given  size  of  wire  the  diameter  Is  taken 
too  large,  the  coils  must  be  stretched  considerably  to  obtain  the  necessary- 
stiffness.  No.  24  (B.  &  S.)  iron  wire  may  be  wound  into  coils  of  y2  in. 
diameter,  while  No.  16  may  be  wound  into  coils  of  from  £4  m-  to  %  in. 
diameter,  and  other  sizes  proportionally.  The  wires  arc  wound  close  on. 


156  ELECTRICAL   DESIGNS 

a  mandrel  in  a  lathe  and  are  stretched  as  they  are  put  in  position.  Fot  the 
larger  sizes  of  wire  a  stretching  of  20  per  cent,  is  sufficient,  while  coils  of 
No.  24  of  6  ins.  or  more  in  length  must  be  stretched  to  about  double  their 
length.  Some  manufacturers  place  asbestos  tubes  inside  coils  of  smaller 
•wire,  which,  as  they  stiffen  the  coils,  permit  coils  of  larger  diameter  and 
reduce  the  stretching  required. 

Dynamo  Field  Rheostats. — Shunt  and  compound-wound  generators 
are  generally  regulated  by  means  of  a  rheostat  in  the  shunt  field  circuit. 
In  Figs.  179  and  180  are  shown  two  dynamo  field  rheostats,  both  of  which 
are  of  fire-proof  construction.  The  form  shown  in  Fig  180  is  intended  for 
small  machines,  while  that  at  Fig.  179  is  adaptable  to  any  size. 


FIG.    179. — DYNAMO    FIELD   RHEOSTAT.  FIF.    l8o. — DYNAMO   FIELD  RHEOSTAT. 

In  the  factory  it  is  generally  easy  to  experimentally  determine  the 
resistance  required  to  cut  down  the  voltage  of  a  machine  to  the  desired 
lower  limit.  Cases  may,  however,  arise,  where  this  is  not  handy,  and  the 
resistance  can  then  be  calculated,  provided  the  excitation-voltage  curve 
of  the  dynamo  and  the  resistance  of  its  field  are  known.  The  calculation 
may  be  illustrated  by  a  practical  example.  The  main  curve  in  Fig.  181 
is  the  excitation-voltage  curve  of  a  i.5~kw  55-volt  generator. 

The  machine  is  run  at  such  a  speed  that  without  any  load  and  without 
I  any  extra  resistance  in  the  field  circuit,  it  generates  65  volts.  A  rheostat 
is  required  which  will  cut  down  the  voltage  to  40.  The  field  resistance  is 
36.4  ohms. 

From  the  curve  we  see  that  at  65  volts  the  exciting  ampere-turns  are 
6,200,  while  at  40  volts  they  are  only  2,300.  The  ampere-turns  are  pro- 
portional to  the  voltage  applied  to  the  shunt.  When  65  volts  are  being 
generated,  the  voltage  at  the  terminals  of  the  shunt  is  65.  At  40  volts  it 

/^  <j 

must,  therefore,  be  65    X — —  =  24.15.     The  rest  of  the  e.  m.  f. 


CONSTRUCTION  AND  CALCULATION  OF  RHEOSTATS 


157 


(40  —  24.15  =  15-85  volts)  must  be  taken  care  of  by  the  drop  in  the 
rheostat.  As  the  same  current  goes  through  field  coil  and  rheostat,  their 
resistance  must  be  to  each  other  as  the  drop  of  potential  in  them.  Thus 


we  get  for  the  resistance  of  the  rheostat  364 


^f=  24  ohms.    The 
24^5 


0.5 


1.5 


2.5 


ft" 


so 

25 

20 
*   15 


Ami  eces 


Imperp  Turn 


0          1000      2000       3000       4000       5000       i 
FIG.    l8l. — EXCITATION  CURVE  OF  1. 5    KW,    55-VOLT  DYNAMO. 


7000      8000 
A  m.Etee. 


largest  current  that  any  part  of  the  rheostat  ever  has  to  carry  is  a  little 

less  than  — •£ — =   1.78  amperes,  and  the  smallest  current — -? : 

364  364  +  24 

=  .65  amperes.  After  finding  a  few  intermediate  points  in  the  same  man- 
ner as  we  found  the  smallest  current,  we  can  draw  a  curve  showing  the 
field  current  for  the  different  voltages.  This  curve  is  also  shown  in  Fig. 
181.  For  small  rheostats,  like  the  one  under  consideration,  but  one  size 
of  wire  is  generally  used.  In  the  present  case  an  iron  wire  No.  22,  or  a 


158  ELECTRICAL   DESIGNS 

German  silver  wire  No.  20  would  have  the  required  current-carrying 
capacity.  The  total  length  of  wire  required  would  be  for  iron  204  ft.,  for 
German  silver  of  the  resistivity  given  above,  148  ft.  In  all  large  rheostats, 
however,  the  size  of  wire  decreases  from  the  "out  "  terminal  to  the  "in" 
terminal.  The  calculation  of  the  different  portions  may  be  illustrated  by 
the  present  example.  Supposing  that  twenty-five  steps  of  about  i  volt 
each  are  desired.  From  the  field-current  curve  and  the  table  of  iron  wire 
we  see  that  the  largest  wire  that  would  be  used  is  No.  22.  Of  this  we 
must  make  the  first  three  sections  of  the  rheostat.  When  the  fourth  sec- 
tion is  inserted  the  field  current  is  reduced  so  much  that  No.  23  will  carry 
it.  The  resistance  of  these  three  sections  is  to  be  calculated  in  the  same 
manner  as  the  total  resistance  of  the  rheostat  was  calculated  above.  We 
then  calculate  the  sections  requiring  No.  23,  No.  24,  etc.,  successively. 


FIG.    l82. — MULTIPLE-CONTACT   RHEOSTAT. 

In  some  lines  of  work  it  is  desirable  to  be  able  to  change  the  e.  m.  f. 
of  a  generator  very  gradually ;  that  is,  by  very  small  steps.  This  requires 
a  large  number  of  contact  points,  and  as  a  rheostat  with  a  large  number 
of  contacts  as  generally  made  (Figs.  179  and  180),  is  quite  expensive  to 
manufacture,  several  types  have  lately  been  brought  out  in  which  an  at- 
tempt is  made  to  simplify  the  construction.  One  of  these  is  illustrated  in 
Fig.  182.  It  consists  of  a  plate  of  slate  in  the  form  of  a  concentric  ring, 
the  inside  and  outside  edges  of  which  are  grooved  to  receive  the  resist- 
ance wire.  The  wire  is  wound  on  a  plate  in  a  continuous  winding  near- 


CONSTRUCTION  AND  CALCULATION  OF  RHEOSTATS 


159 


ly  all  around  the  ring,  as  seen  in  the  illustration.  The  plate  carrying  the 
wire  is  clamped  between  two  other  plates  of  slate.  The  front  plate  carries 
the  contact  lever,  while  to  the  back  plate  are  fastened  two  strips  of  brass 
by  means  of  which  the  rheostat  is  fastened  to  the  switch-board.  The 
sliding  contact  piece  bears  directly  on  the  wire.  The  rheostat  illustrated 
has  1 80  steps. 

Fig.  183  shows  diagrammatically  a  rheostat  in  which  the  number  of 
steps  is  equal  to  twice  the  number  of  contacts  less  one.  It  consists  of  an 
ordinary  rheostat  with  a  slightly  different  contact  arrangement.  The 
contact  lever  carries,  in  addition  to  the  regular  contact  piece,  another 


0 


6     6 


Am.Elec.  Am.Elec. 

FIG.    183. — MULTIPLE  CONTACT  RHEOSTAT.      FIG.    185. — MOTOR-REGULATING  RHEOSTAT. 

double  contact  piece  which  is,  however,  insulated  from  it.  The  main  con- 
tact piece  is  wider  than  the  distance  between  two  neighboring  contact 
points,  while  the  two  prongs  of  the  double  contact  piece  are  narrower 
than  this  distance.  Suppose  that  the  current  enters  through  the  contact 
lever.  It  will  then  pass  from  contact  2  to  contact  3  through  the  sections, 
a1  a  and  fr1  b  in  parallel,  and  from  3  through  the  rest  of  the  sections  in 
series.  The  resistance  of  two  sections  in  parallel  is,  of  course,  equal  to 
one-half  the  resistance  of  a  single  section.  When  the  lever  is  turned  to 
the  right,  the  main  contact  piece  comes  in  contact  with  3  and  the  prongs 


160  ELECTRICAL  DESIGNS 

of  the  extra  contact  piece  are  between  contacts  i  and  2,  and  2  and  3  re- 
spectively. The  parallel  resistance  is  then  cut  out.  This  type  was  sug- 
gested by  Vedovelli. 

Motor-Starting  Rheostats. — When  a  shunt  motor  is  started  up,  a  re- 
sistance must  be  placed  in  its  armature  circuit,  to  prevent  an  abnormal 
rush  of  current.  This  resistance  is  cut  out  stepwise  as  the  motor  gains 
speed.  Motor  starters  have  generally  but  one  size  of  wire  all  through, 
of  sufficient  cross  section  to  carry  the  full  load  current  for  one  minute. 
(See  table  below. )  The  resistance  of  the  rheostat  should  be  such  that 
when  it  is  connected  across  the  mains,  a  current  equal  to  the  full  load  cur- 
rent of  the  motor  will  pass.  From  this  condition  the  resistances  of  start- 
ing rheostats  for  motors  of  different  outputs  and  voltages,  compiled  in 
the  following  table,  have  been  calculated.  (Ten  per  cent,  is  allowed  for 
armature  and  friction  loss  in  the  motor.) 

RESISTANCE   IN   OHMS    OF   MOTOR-STARTING   RHEOSTATS. 

HP.  I  3  5  7  10  15         20  30  40  50 

iiov  15  5  3  2.1           1.5           I             .75  .5  .37  .3 

220  Co  20  12  8.4          6             43  2  1.5  1.2 

500  300  ioo  60  42  30  20        15  10  7-5  6 

Motor-starting  rheostats  are  nearly  always  automatic;  that  is,  they 


FIG.    184. — AUTOMATIC    STARTING   RHEOSTAT. 

have  an  electro-magnetic  attachment  by  means  of  which  the  armature  is 
automatically  cut  out  of  circuit  whenever  the  main  current  fails  for  any 
reason.  This  protects  the  motor  from  injury  when  the  current  in  the 
mains  is  established  again.  Fig.  184  shows  a  much-used  type  of  auto- 
matic starting  rheostat.  The  rheostat  has  an  electromagnet  on  its  face 
plate ;  the  coil  of  this  magnet  is  in  series  with  the  field  winding  of  the 
motor.  The  contact  lever  is  of  iron  and  has  a  spiral  spring  inside  its  hub. 
The  magnet  holds  the  lever  in  position  when  the  resistance  of  the  rheo- 


CONSTRUCTION  AND  CALCULATION  OF  RHEOSTATS  l6l 

stat  is  cut  out  of  circuit,,  but  when  the  current  in  the  field  circuit  ceases 
the  lever  is  brought  back  to  the  dead  button  by  the  action  of  the  spring. 

Motor-Regulating  Rheostats. — The  speed  of  motors  may  be  regulated 
by  means  of  a  rheostat  in  the  armature  circuit.  For  the  special  case  of  a 
constant  torque  on  the  motor,  the  speed  is  proportional  to  the  counter 
e.  m.  f.  and  the  current  remains  constant,  both  for  shunt  and  series 
motors.  The  resistance  necessary  to  reduce  the  speed  to  a  certain  frac- 
tion of  its  original  value  may  be  found  by  the  following  rule  : 

Multiply  the  e.  m.  f.  of  the  mains  minus  the  drop  in  armature  (and 
field  in  case  of  series  motors),  by  the  difference  of  unity  and  the  given 
fraction,  and  divide  the  product  by  the  current.  The  quotient  obtained  is 
equal  to  the  required  resistance  in  ohms. 

Motor-regulating  rheostats  are  also  made  in  which  the  contact  lever 
is  automatically  held  in  any  position  and  automatically  released  in  case 
of  overload  or  when  the  main  current  is  interrupted.  Fig.  185  shows  a 
front  view  of  such  a  rheostat.  Two  levers  are  rotatable  on  a  stud  fast- 
ened to  the  base  plate.  One  of  these,  the  contact  lever,  A,  has  an  el- 
bow-shaped side  projection,  L,  with  a  number  of  notches  on  its  outer 
edge,  corresponding  to  the  contact  points.  The  other  lever,  B  Bt  is  of 
iron,  and  is  two-armed.  The  lower  arm  serves  as  armature  to  the  elec- 
tromagnet, M.  The  other  arm  has  pivoted  to  it  at  its  extremity  a  third 
lever,  C,  provided  with  a  catch,  c. 

The  two  arms  of  lever  C,  are  of  unequal  moment  and  one  of  the 
arms  rests  continuously  against  a  stud,  s,  fastened  to  the  base  plate.  A 
helical  spring  (not  seen  in  diagram)  coiled  around  the  hub  of  the  lever  A, 
holds  the  lever  against  the  stop,  S,  when  the  rheostat  is  not  in  use. 
When  the  lever,  A,  is  turned  to  the  right,  lever  B  also  turns  on  account 
of  the  friction  between  the  two  levers,  and  is  thus  brought  near  the  poles 
of  the  magnet,  M.  The  catch,  c,  engages  into  the  notch  corresponding 
to  the  contact  point,  on  which  the  contact  lever  is  left.  The  catch  is 
locked  by  the  electromagnet  through  the  armature,  B.  The  magnet,  M, 
is  wound  differentially,  one  winding  being  in  the  field  and  one  being  in 
the  armature  circuit  of  the  motor.  The  turns  of  the  windings  are  so  pro- 
portioned that  under  ordinary  conditions  the  effect  of  the  field  current 
predominates.  When  the  current  in  the  armature  circuit  rises  above  a 
certain  value,  the  magnet  is  so  much  weakened  that  it  cannot  resist  the 
action  of  the  spring  on  the  hub  of  lever  A.  It  leaves  go  its  armature,  B, 
the  catch,  c,  disengages,  and  lever  A,  returns  to  the  "off"  button.  The 
same  happens  if  the  main  or  the  field  circuit  should  be  opened. 


CHAPTER  XIX. 


SIMPLE  VOLTMETERS,   AMMETERS,  AND  WATTMETERS. 


Nearly  all  forms  of  meters  depend  upon  the  magnetic  effects  of 
the  current  for  their  action.  These  may  be  divided  into  solenoid  in- 
struments, magnetic  vane  or  needle  instruments,  and  moving  coil  in- 
struments. Another  class  of  instruments  of  great  importance  uses  the 
heating  effect  of  the  current,  which  produces  expansion  in  a  strip  of 
metal  or  a  wire,  as  the  source  of  their  indications. 


FIGS.    186  AND    187. — SECTION  AND  ELEVATION,    MAGNETIC   VANE  AMMETER. 

A  very  satisfactory  instrument  is  shown  by  Figs.  186  and  187. 
Here  the  fact  that  the  lines  of  magnetic  force  crowd  close  together 
along  the  inner  sides  of  a  solenoid  is  used  as  the  principle  of  action.  The 
coil  of  large  wire  is  wound  on  a  brass  tube  with  wooden  or  fiber  heads, 
one  end  of  the  tube  being  closed  with  a  brass  plug,  Q.  A  piece  of 
brass,  A,  is  soldered  to  the  other  end  of  the  tube,  and  through  this 
and  the  plug  are  screws,  5"  and  Sl,  coned  out  for  the  reception  of  the 
pointed  ends  of  the  pivot,  as  shown.  The  screws  must  be  of  brass  or 
other  non-magnetic  material.  They  are  not  arranged  in  the  center  of 
the  tube,  but  are  a  little  above  it,  say  5-32  in.,  if  the  tube  is  an  inch  in 


SIMPLE   VOLTMETERS,  AMMETERS  AND  WATTMETERS        163 

diameter.  On  the  pivot,  which  is  of  hard  steel,  is  mounted  a  little  vane 
or  wing  of  thin  sheet  iron  (the  sort  used  by  photographers  for  the  basis 
of  "tin-types"  is  best)  bent  to  the  shape  shown  in  the  small  detail  draw- 
ing. This  should  have  about  the  relative  size  shown  in  the  illustration. 
The  needle,  P,  is  of  aluminum  wire,  for  the  sake  of  lightness,  and  the 
whole  is  so  balanced,  by  soldering  on  bits  of  copper  wire  if  necessary, 
that  it  hangs  normally  as  shown  in  Fig.  187.  The  vane  being  eccentric 
to  the  tube  carrying  the  coil  tends  to  approach  its  inner  surface  when 
current  passes.  It  must  be  so  attached  to  the  pivot  that  this  tendency 
causes  the  needle  to  sweep  over  the  scale,  M.  As  in  the  instrument  just 
described  this  scale  is  of  paper  mounted  on  a  scrap  of  looking-glass. 
The  whole  is  attached  to  a  circular  wooden  base  and  forms  a  convenient 
wall  or  switchboard  instrument.  A  cover  to  exclude  dust  and  keep  off 
stray  air-currents  would  be  a  valuable  addition. 

This  ammeter  may  be  made  very  sensitive  and  accurate  if  care  is 
taken  in  its  construction.  The  lighter  the  needle  the  more  sensitive  the 
instrument,  other  things  being  equal.  To  decrease  its  sensitiveness 
the  lower  part  of  the  needle  system  should  be  loaded  so  as  to  bring  the 
center  of  gravity  of  the  whole  lower  and  thus  cause  a  greater  tendency 
for  the  needle  to  return  to  its  zero  position. 

The  ammeter  may  be  converted  into  a  voltmeter  by  the  use  of  a  fine 
wire  high-resistance  coil  of  many  turns  in  place  of  the  coarse  coil 
shown.  For  such  an  instrument,  used  as  an  ammeter,  and  measuring 
currents  up  to  100  amperes,  about  eighteen  or  twenty  turns  of  wire  /4-in. 
in  diameter  will  be  found  sufficient. 

There  are  many  cases  where  it  is  desirable  to  know  the  direction  of 
the  current  as  well  as  its  volume.  Neither  of  the  instruments  described 
indicates  this.  The  next  ammeter  to  be  described  not  only  indicates  the 
direction  and  amount  of  the  current,  but  also  possesses  two  valuable 
qualities  not  shared  by  the  cruder  forms  described — portability  and  free- 
dom from  vibration  of  the  needle.  In  other  words,  it  is  a  "dead-beat"  in- 
strument, the  needle  going  promptly  to  its  place  on  the  scale  and  stop- 
ping without  vibration.  It  is  a  very  satisfactory  and  useful  instrument 
and  will  be  described  at  some  length  on  account  of  its  various  good 
qualities. 

The  ring,  R  (Figs.  188  and  189),  is  made  of  good  quality  tool  steel, 
I  in.  x  y2  in.,  and  bent  around  a  diameter  of  43^  ins.  The  ends  do  not 
meet,  but  are  rounded  off  as  shown  by  dotted  lines  in  Fig.  189,  leaving 
an  opening  about  I  in.  between  them.  After  this  ring  has  been  forged 
into  shape  and  finished  by  the  rounding  of  the  two  ends  and  the  boring 
of  the  two  holes  for  the  screws,  m  and  n,  it  is  hardened  by  heating  it 


i64 


ELECTRICAL  DESIGNS 


SIMPLE   VOLTMETERS,  AMMETERS   AND  WATTMETERS        165 

red  hot  and  suddenly  cooling-  it  in  water,  or  better,  a  solution  of  sal 
ammoniac.  It  is  then  magnetized  by  wrapping  it  with  about  seventy- 
five  turns  of  wire  and  passing  a  strong  current,  or  by  rubbing  it 
on  the  poles  of  a  dynamo,  and  after  magnetization  it  is  boiled  for  an 
hour  in  water.  Then  it  should  be  magnetized  again,  and  again  boiled, 
this  being  repeated  several  times,  the  magnetizing  always  being  done 
in  the  same  way  and  never  reversed.  By  this  method  of  magnetizing 
and  boiling  the  ring  is  brought  to  a  permanent  state  and  does  not  lose 
its  magnetism,  as  would  be  the  case  if  no  such  precaution  were  taken. 

The  tube,  T,  shown  also  in  one  of  the  small  detail  drawings,  is  of 
brass,  I  in.  outside  diameter  and  about  1-16  in.  thick  and  4^  ins.  long. 
In  the  middle  of  it  is  cut  an  opening  y2  in.  wide  by  ^  m-  long")  as  shown 
in  the  small  drawing.  This  is  for  the  withdrawal  of  the  iron  needle 
described  below.  Referring  to  Fig.  188,  which  shows  a  cross  section  of 
the  instrument  on  the  line  A  B  (Fig.  189),  it  is  seen  that  the  tube  is  sup- 
ported between  small  castings  of  brass  which  are  clamped  between  the 
poles  of  the  permanent  ring  magnet  by  the  screws  m  and  n.  To  the 
lower  of  these  castings  the  tube  is  soldered  at  its  middle,  the  opening 
already  described  being  on  the  upper  side,  as  shown  at  Q,  Fig.  189. 
The  upper  casting  clamps  the  tube  solidly  in  place  when  the  two  screws, 
m  and  n,  are  drawn  up  tight. 

Through  these  two  castings  are  tapped  brass  screws  coned  out  for 
the  reception  of  the  pivot  carrying  the  needle,  N.  This  pivot  is  of  hard 
steel  wire,  the  ends  being  coned  in  a  small  lathe  or  ground  off  to  shape 
on  an  emery  wheel.  On  it  is  mounted  the  soft  iron  needle,  N,  shown 
in  perspective  in  one  of  the  small  drawings.  This  should  be  of  the 
softest  and  purest  iron  obtainable,  $  in.  long  over  all,  about  ^  in.  wide 
in  the  middle,  and-  about  the  same  thickness,  measured  along  the  pivot. 
It  must  be  filed  carefully  into  perfectly  symmetrical  shape.  Attached 
to  the  pivot  is  the  aluminum  pointer,  P,  which  sweeps  over  a  scale  and 
mirror  as  described  above.  It  will  be  noticed  that  the  zero  point  of  this 
scale  is  at  the  middle,  the  needle  being  deflected  in  either  direction,  ac- 
cording to  the  direction  of  the  current. 

In  the  ends  of  the  brass  tube,  T,  are  soldered  two  plugs  carrying 
the  soft  iron  screws,  S  and  5"1.  These  are  intended  for  the  regulation 
of  the  scale  of  the  instrument,  and  once  adjusted  are  to  be  left  alone. 
By  screwing  them  in  nearer  the  needle,  AT,  the  instrument  becomes  more 
sensitive,  that  is,  it  gives  a  larger  deflection  for  the  same  current. 
Hence,  when  the  instrument  is  assembled,  the  maximum  current  it  is 
intended  to  register  should  be  sent  through  it  and  screws  adjusted  until 
the  needle  is  at  its  extreme  deflection. 


366  ELECTRICAL  DESIGNS 

To  soften  the  iron  screws,  5*  and  S\  put  them  in  a  small  sand 
crucible  and  cover  them  with  powdered  lime.  Then  heat  the  crucible 
to  a  cherry  red  heat  in  a  charcoal  or  anthracite  fire,  leaving  it  to  cool 
very  slowly  as  the  fire  dies  down.  The  lime  prevents  the  formation  of  a 
scale  of  oxide  on  the  screws,  which  may  be  cleaned  after  they  are  cool 
by  dropping  them  for  a  moment  in  weak  muriatic  acid  and  washing  in 
water  containing  a  little  ammonia. 

The  coil  is  wound  as  shown  on  the  brass  tube,  its  ends  being  at- 
tached to  appropriate  binding  posts.  The  figure  shows  a  coil  of  only 
twelve  turns,  intended  for  the  measurement  of  fairly  large  currents,  but 
for  small  currents  the  wire  may  be  smaller  and  the  turns  more  in 
proportion.  For  currents  up  to  15  amperes  use  No.  10  wire  and  put  on 
sixty  turns,  thirty  on  each  end  of  the  tube. 

The  ring  magnet,  R,  is  fastened  down  to  the  wooden  base  by 
clamps,  K.  As  shown  in  the  illustration  the  instrument  is  a  table  form, 
adapted  for  use  in  a  horizontal  position  only.  If  the  needle  system  shown 
in  the  small  drawing  is  balanced  perfectly  for  all  positions  (which 
would  require  a  small  counterweight  to  compensate  for  the  pointer,  F), 
the  instrument  may  be  used  in  any  position. 

For  use  as  a  voltmeter  the  coarse  coils  shown  should  be  replaced 
by  coils  of  fine  wire  wound  on  insulating  spools  and  slipped  over  the 
ends  of  the  spool.  Indeed,  a  combination  instrument  may  be  made 
by  slipping  these  spools  on  over  the  cammeter  winding  as  shown.  For 
the  best  results  these  spools  should  contain  the  largest  possible  amount 
of  the  finest  possible  wire,  and  should  be  connected,  in  addition,  through 
a  resistance  in  the  base  of  the  instrument,  care  being  taken  to  so  wind 
the  latter  that  it  does  not  produce  any  magnetic  effects.  The  total  re- 
sistance of  the  coil  and  the  additional  resistance  for  measurements  up 
to  150  volts  should  not  be  less  than  9,000  or  10,000  ohms.  For  higher 
voltage  additional  resistance  or  a  shunt  must  be  used.  Unless  the  re- 
sistance is  very  high  so  much  current  will  flow  through  the  coils  that 
they  will  heat  or  even  burn  out  if  the  instrument  is  left  in  circuit,  hence 
the  necessity  for  care  in  providing  enough  resistance.  It  is  also  to  be 
noted  that  a  voltmeter  is  exposed  to  the  full  pressure  of  the  current  that 
it  is  measuring,  and  that  its  insulation  cannot  be  too  careful.  In  an 
ammeter  the  fall  of  potential  is  utterly  negligible  and  insulation  is  not 
a  feature  of  particular  importance,  but  with  a  voltmeter  this  is  different, 
.a  short  circuit  leading  instantly  to  disastrous  results. 

The  above  instrument,  which  is  similar  to  those  made  by  Car- 
pentier  and  Ayrton  &  Perry,  is  a  thoroughly  satisfactory  shop  meter,  if 
carefully  and  accurately  made.  Unfortunately,  like  the  other  instru- 
ments described  above,  it  is  useless  for  alternating  currents. 


SIMPLE.  VOLTMETERS,  AMMETERS  AND  WATTMETERS        167 

The  instrument  shown  in  Figs.  190  and  191  belongs  to  the  moving 
coil  type  and  is  adapted  to  cither  alternating  or  direct  currents.  On  a 
wooden  base  are  mounted  two  wooden  rings,  R  R,  about  6  ins.  in  diam- 
eter, upon  which  are  wound  about  20  turnc  cf  No.  10  wire.  The  blocks, 
17  and  IV  carry  on  their  upper  surfaces  small  copper  cups,  A  and  A  i, 
which  are  soldered  to  the  copper  strips,  m  and  n,  and  so  connected  that 
the  current  circulating  in  the  coils,  7^  R,  includes  in  its  circuit  the  two 
cups  and  the  coil,  C,  which  is  suspended  between  them.  This  coil  is 
wound  on  a  thin  hardwood  frame  of  the  shape  shown  in  the  draw- 
ings, the  upper  of  the  two  small  drawings  showing  its  upper  surface. 
The  coil  has  20  turns  of  No.  10  wire,  its  ends  being  soldered  to  the 
copper  pins,  K  K.  One  of  these  is  shown  on  a  larger  scale  in  the  small 
drawings.  At  their  bottoms  they  are  filed  into  knife-edges,  so  that 
when  they  are  placed  in  the  copper  cups,  A  and  Ait  the  coil,  C,  rocks  on 


FIGS.    190  AND    IQI. 

them.  The  bottoms  of  the  copper  cups  are  slightly  hollowed  to  keep 
these  knife-edges  in  place  at  the  center,  and  the  whole  system  is  balanced 
so  that  it  rocks  very  easily  on  the  knife-edges.  The  cups,  A,  and,  Ait  are 
filled  about  half  full  of  mercury,  on  the  top  of  which  is  placed  a  drop 
cf  kerosene,  or,  better,  of  a  mixture  of  about  4  parts  of  kerosene  and  i 
of  alcohol  in  which  is  dissolved  5  per  cent,  of  cyanide  of  potash.  This 
preserves  the  mercury  surface  from  oxidation  and  also  relieves  the 
surface  tension.  A  pointer,  P,  is  attached  to  the  rocking  coil  and  plays 
in  front  of  a  scale  as  shown.  The  instrument  should  be  covered  with 
a  bell-glass,  or  a  wooden  box  having  a  glass  front,  to  keep  out  dust  and 
air  currents. 

While  this  is  by  no  means  an  ideal  instrument  it  is  a  satisfactory  one 
for  many  purposes.  It  should  be  mounted  on  a  shelf  on  a  wall  or  in 
some  other  place  where  it  will  not  be  disturbed  or  subject  to  vibrations, 


1 68  ELECTRICAL   DESIGNS 

and  it  should  be  taken  down  and  cleaned  occasionally.  The  nearer  th'e 
knife-edges  come  to  the  center  of  gravity  of  the  suspended  system  the 
more  sensitive  the  instrument  will  be.  It  is  advisable  to  make  the  coil, 
C,  as  light  as  possible,  and  for  this  reason  aluminum  wire  should  be 
used  in  place  of  copper  for  winding  it  if  this  can  be  obtained.  This  in- 
strument is  not  "dead  beat,"  but  it  has  a  fairly  constant  scale  through  a 
range  of  about  30  degs.,  which  is  a  great  advantage. 

For  alternating  current  work  the  "hot-wire"  instruments  possess 
some  important  advantages,  among  them  that  of  not  affecting  the  circuit 
in  any  way,  as  the  coil  instruments  do  by  their  self-induction.  The  in- 
strument next  to  be  described  is  somewhat  similar  to  the  Cardew  hot- 
wire volt-meter,  and,  while  rather  delicate,  is  an  excellent  instrument  in 
careful  hands.  Figs.  192  and  193  are  front  and  side  elevations  of  it, 
partly  in  section. 

The  principle  upon  which  it  depends  is  the  expansion  of  a  small 
wire  heated  by  the  current  to  be  measured.  A  wooden  box,  8  ins.  in 
diameter,  is  turned  to  form  the  head  of  the  instrument,  and  to  this  is  at- 
tached as  shown  a  piece  of  iron  gas-pipe,  T,  i  1-4  ins.  inside  diameter 
and  about  3  ft.  long.  At  the  bottom  of  this  pipe  (the  instrument  is  in- 
tended to  be  attached  to  a  wall  with  the  wooden  box  part  uppermost)  is  a 
brass  plug,  insulated  from  the  pipe  by  an  insulating  bushing,  B,  and 
carrying  a  threaded  rod  and  nut,  N.  This  threaded  rod  is  grooved,  a 
small  pin  engaging  in  the  groove  so  that  when  the  nut,  AT,  is  turned  the 
rod  is  screwed  in  or  out,  but  does  not  turn  around.  There  should  not 
be  the^  least  lost  motion  about  this  fitting,  as  the  motion  of  the  threaded 
rod  must  be  accurate  and  exact.  In  the  upper  extremity  of  the  threaded 
rod  is  soldered  a  small  bit  of  copper  wire  split  at  its  upper  end.  This  is 
to  clamp  the  platinum  wire,  w,  whose  expansion  is  recorded  by  the  in- 
strument. 

At  the  upper  end  of  the  tube,  T,  is  arranged  a  small  drum,  R,  having 
coned  ends  resting  in  the  screws  as  shown,  or  the  screws  may  be  pointed 
and  the  drum  coned  out  for  their  reception.  The  drum  should  be  not 
more  than  3-16  in.  in  diameter,  and  is  best  made  of  steel.  It  is  shown 
relatively  too  large  in  the  illustrations.  Around  this  drum  the  platinum 
wire,  iv,  is  wrapped  two  or  three  times^  the  surface  of  the  drum  being 
first  enameled  with  a  mixture  of  finely  powdered  asbestos  and  water- 
glass  (soda  silicate)  painted  on  and  dried  with  gentle  heat.  It  is  not 
absolutely  necessary  to  enamel  the  surface,  but  it  is  well  to  do  so,  as  this 
prevents  any  current  from  flowing  through  the  cone  bearings  of  the 
drum  and  heating  them. 

In  the  tube,  T,  is  secured  a  small  piece  of  1-2  in.  iron  pipe,  closed  at 


SIMPLE   VOLTMETERS,  AMMETERS   AND  WATTMETERS        169 


FIGS.    192    AND    193. — HOT  WIRE  VOLTMETER. 


the  bottom  with  a  plug  and  partly  filled  with  mercury.     The  platinum 
wire,  after  passing  around  the  drum,  is  deflected  by  the  glass  rod,  G, 


17°  ELECTRICAL  DESIGNS 

so  that  the  wire  attached  to  the  weight,  A,  suspended  from  the  platinum 
wire,,  dips  in  the  mercury.  This  wire  should  be  of  copper  and  the 
weight  should  be  of  lead  or  brass,  its  weight  depending  upon  the  size 
of  the  platinum  wire,  but  enough  to  keep  it  taut.  For  a  No.  38  platinum 
wire  the  weight  should  be  about  15/2  ozs. 

A  resistance  is  absolutely  necessary  with  this  type  of  voltmeter,  and 
the  otherwise  empty  space,  S,  will  contain  it  comfortably.  For  voltage 
up  to  150  a  resistance  of  about  4,000  ohms  should  be  used  and  the  pla- 
tinum wire  should  be  about  No.  38  or  40.  The  resistance  can  be  wound 
on  a  wooden  spool  and  is  best  of  German  silver  wire,  about  No.  32,  pre- 
ferably not  smaller. 

As  the  expansion  of  the  iron  pipe  and  the  platinum  wire  is  not  the 
same,  the  daily  changes  of  temperature,  the  stretch  of  the  fine  wire,  etc., 
will  make  the  zero  point  somewhat  uncertain.  For  this  reason  the  nut, 
N,  and  its  adjuncts  are  provided,  so  that  the  needle  can  always  be  ad- 
justed to  zero  before  the  reading  is  made.  The  instrument  is  quite  sensi- 
tive, absolutely  dead-beat,  and  entirely  unaffected  by  external  magnetism. 
The  best  way  to  increase  its  accuracy  without  making  it  more  delicate 
is  to  lengthen  the  pipe,  T,  but  this  has  practical  disadvantages.  It  should 
be  mounted  by  means  of  wooden  cleats  against  a  wall  and  left  there. 
Care  should  be  taken  in  winding  the  resistance  coil  not  to  make  it  induc- 
tive, as  this  will  vitiate  the  accuracy  of  the  instrument  and  introduce 
complications  in  the  circuit.  To  wind  the  resistance  non-inductively 
two  wires  should  be  wound  at  once  on  the  spool,  and  then  connected 
together  so  that  the  current  circulates  in  different  directions  through 
each. 

A  sensitive  and  excellent  wattmeter  may  be  very  simply  made  as 
follows  :  On  a  wooden  base  build  up  with  brass  screws  or  glue  a  wooden 
box,  as  shown  in  Figs.  194  and  195,  about  7  ins.  X  8  ins.,  and  7^2  ins. 
high,  having  a  glass  front,  G.  Against  the  back  of  this  box  is  mounted 
a  wooden  spool,  At  carrying  a  coil  of  a  few  turns  of  coarse  wire.  For 
currents  up  to  about  20  amperes  No.  8  wire  and  8  or  10  turns  are  ad- 
visable. In  the  center  of  the  top  of  the  box  is  cut  a  hole,  and  mount- 
ed above  this  in  a  wooden  ring,  R,  is  an  ordinary  glass  lamp  chimney 
of  the -shape  shown  (such  as  is  used  with  "student  lamps").  In  the 
top  of  this  is  a  plug,  H  fitting  so  that  it  can  be  turned  with  the  fingers, 
and  carrying  two  wires,  m,  n,  fitting  rather  tightly  in  the  fibre  or  wooden 
plug.  If  wood  is  used  it  should  be  boiled  in  paraffine,  as  the  voltage 
measured  is  in  full  force  between  the  two  pins,  m  and  n.  These  pins 
should  be  J^  in.  apart,  and  should  be  split  at  the  bottom  so  that  the 
two  thin  suspension  wires,  5  and  S,  may  be  pinched  in  the  split  part. 


SIMPLE   VOLTMETERS,  AMMETERS   AND  WATTMETERS        171 


Suspended  from  these  two  wires  is  the  coil  of  fine  wire,  K.  This  is 
wound  on  a  fibre  spool,  2  ins.  outside  diameter,  I  in.  inside  diameter, 
and  having  a  space  for  the  winding  y2  in.  wide  measured  along  the  axis 
of  the  spool.  It  should  be  as  thin  as  possible  to  reduce  its  weight,  and 
should  contain  about  300  turns  of  No.  32  wire.  In  winding  this  the 
greatest  care  should  be  used  to  insulate  it  thoroughly,  and  the  wire 

H 


FIG.    194. — SIDE   ELEVATION,  WATTMETER.  FIG.    IQ5. — END   ELEVATION,    WATTMETER. 

should  be  copiously  shellacked  and  the  coil  thoroughly  baked  after  com- 
pletion. The  ends  of  this  coil  are  connected  to  the  two  thin  sheet  copper 
or  aluminum  strips,  c,  which  are  shellacked  to  the  sides  of  the  spool. 
At  the  lower  end  of  each  of  these  strips  is  a  vane  or  projection,  vt  the 
two  being  arranged  to  point  in  opposite  directions.  These  are  intended 
to  dip  into  kerosene  oil  contained  in  the  cup,  0,  and  to  dampen  the 
swinging  of  the  suspended  coil.  Across  the  two  strips,  C,  is  fastened 
with  gummy  shellac  a  small  mirror,  M,  best  made  by  silvering  the  sur- 
face of  a  bit  of  glass,  such  as  is  used  for  microscope  slide  covers,  though 


172 


ELECTRICAL  DESIGNS 


a  piece  of  ordinary  thin  looking  glass  will  answer.  The  two  suspension 
wires,  which  should  be  of  exactly  equal  length,  are  fastened  to  the  strips, 
c,  with  small  drops  of  solder.  These  suspension  wires  should  be  of 
copper,  not  larger  than  No.  36  gauge  and  preferably  smaller.  It  is  not 
necessary  to  remove  the  insulation  from  them. 

Two  binding  posts,  P  and  Pi,  are  provided  for  attaching  the  fine 
wires,  w,  leading  from  the  pins,  m  and  n.  To  these  posts  are  connected 
the  terminals  from  the  two  sides  of  the  circuit  to  be  measured,  while  the 
main  current  is  led  through  the  coil,  A.  The  deflections  of  the  swinging 
coil  are  read  off  by  means  of  a  lamp  and  scale,  a  simple  and  excellent 
I  arrangement  for  this  purpose  being  described  by  Mr.  J.  F.  Hobart  in 
Chapter  XXI.  For  small  deflections  the  scale  readings  are  proportional 

to  the  watts.  This  instrument  is 
equally  good  for  alternation  and 
direct  current  work,  and  in  careful 
hands  is  surprisingly  accurate.  It 
is  quite  good  enough  for  incandes- 
cent lamp  measurements. 

Of  course,  a  resistance  must 
be  used  in  series  with  the  fine  coil, 
the  amount  being  proportional  to 
the  voltage  of  the  circuit.  Up  to 
150  volts,  using  No.  32  wire  on  the 
coil,  the  resistance  should  be  about 
2,500  ohms,  and  it  should  be 
wound  non-inductively  as  described 
above.  If  very  accurate  determin- 
ations are  to  be  made  the  self- 
induction  wire  circuit  may  be  com- 
pensated by  a  condenser  consisting 
of  glass  plate  with  a  strip  of  tin  foil 
on  each  side,  but  this  is  an  unneces- 
sary refinement  for  ordinary  work. 
A  watt-hour  meter  may  be  made  from  any  pendulum  clock,  a  good 
one  being,  of  course,  preferable.  The  arrangement  is  shown  in  Fig.  196. 
The  bob  of  the  pendulum  is  replaced  by  a  fine  wire  coil  of  many  turns, 
small  wires  being  led  up  to  points  near  the  center  of  motion  of  the 
pendulum  and  thence  to  the  binding  posts  on  the  case.  Below  the 
pendulum  and  as  near  it  as  possible  is  a  coarse  wire  coil,  B.  The  main 
current  flows  through  this  while  the  fine  wire  swinging  coil  is  bridged 
across  the  circuit^  The  clock  is  first  adjusted  carefully  to  keep  correct 


FIG.    196. — RECORDING  WATTMETER. 


SIMPLE   VOLTMETERS,  AMMETERS   AND  WATTMETERS        173 

time  with  no  current  in  the  coils.  The  mutual  attraction  between  the 
coils  causes  either  an  acceleration  or  retardation  of  the  rate  of  the  clock 
in  proportion  to  the  watts  passing,  and  all  that  is  necessary  is  to  know 
the  constant  of  the  apparatus  and  note  the  gain  or  loss  of  the  clock.  Up 
to  a  gain  or  loss  of  about  3  minutes  per  hour  this  meter  is  very  accurate. 
Of  course,  to  get  the  constant,  it  is  necessary  to  calibrate  the  meter  by 
working  it  for  several  hours  on  a  known  load.  It  is  equally  applicable 
to  direct  or  alternating  currents. 

On  a  constant  pressure  supply  system,  direct  current,  the  swinging 
coil  may  be  replaced  by  a  bar  magnet,  and  the  instrument  then  becomes 
a  coulomb-hour  meter,  or,  if  the  voltage  is  constant,  a  watt-hour  meter. 
The  well  known  Aron  meter,  much  used  in  England,  is  constructed  on 
this  principle. 


CHAPTER    XX. 


D'ARSONVAI,  GALVANOMETER. 


Of  the  several  types  of  galvanometers,  there  is,  perhaps,  no  other 
which  covers  so  wide  a  range  of  usefulness  as  the  D'Arsonval,  and  cer- 
tainly there  is  no  other  which  can  take  its  place  in  the  dynamo  room, 
for,  owing  to  its  intense  magnetic  field,  it  can  be  used  in  close  proximity 
to  dynamos,  and  it  is  not  affected  by  wires  carrying  heavy  currents,  as  are 
other  types.  It  is  a  dead-beat  instrument,  enabling  readings  to  be  taken 
very  rapidly,  and  it  has  not  the  delicate  suspension  of  other  forms, 
making  it  a  convenient  portable  instrument.  The  form  which  it  is  the 
purpose  of  this  article  to  describe  will  be  found  to  meet  the  require- 
ments of  all  but  the  most  delicate  tests,  when,  of  course,  a  high-priced 
instrument  is  essential. 

The  magnet  for  this  instrument  is  built  up  from  sheets  1-16  in. 
steel  of  the  form  shown  in  Fig.  197.  Six  of  these  plates  arc  bolted  to- 
gether, making  the  complete  magnet  3-8  in.  thick.  The  plates  should 
be  cut  or  forged  from  the  best  steel  and  bolted  together;  while  in  that 
position  they  should  be  carefully  finished,  after  which  they  may  be  taken 
apart  and  tempered,  but  care  should  be  taken  to  mark  them  with  a 
prick  punch  so  that  they  may  be  reassembled  in  the  same  relative  posi- 
tions. To  harden  them,  heat  a  large  flat  piece  of  iron  and  lay  one  of  the 
pieces  on  this.  When  it  becomes  a  cherry  red,  quickly  plunge  it,  points 
first,  into  a  pail  of  water,  repeating  the  operation  with  each  of  the 
remaining  pieces.  After  being  hardened  they  may  be  magnetized  byi 
means  of  the  usual  coils  about  their  poles.  Each  should  be  magnetized  A 
separately.  They  may  then  be  reassembled  and  polished. 

A  base  should  be  provided  of  hard  rubber  or  well  seasoned  hard 
wood,  and  should  be  turned,  as  shown  in  Figs.  197  and  199.  Cut  a 
mortise  in  the  center  which  shall  be  a  snug  fit  for  the  magnet  and  allow 
it  to  go  J4  m-  deep.  Cut  two  strips  of  1-16  in.  brass,  3-8  in.  by  5  3-8  ins., 
and  at  one  end  of  each  turn  up  3-8  ins.,  forming  a  right  angle.  Drill 
through  the  short  limb  of  each  for  a  screw  to  fasten  it  to  the  base ;  at 


D'ARSONVAL    GALVANOMETER 


175 


2  7-8  ins.  from  the  base  of  these  L-shaped  pieces  drill  a  3-16  in.  hole  and 
bolt  one  to  the  back  of  each  limb  of  the  magnet.  Place  the  magnet  in 
position  and  fasten  by  screws  to  the  short  limbs  of  the  brass  strips. 


iimfiMi|| 


FIG.    19.7. 


A  pattern  must  be  made  for  the  standard  shown  in  Fig1.  198,  and 
one  cast  from  brass.  It  will  be  well  to  have  this  y2  in.  longer  than 
shown,  as  experience  indicates  that  the  suspension  is  improved  by  being 


176 


ELECTRICAL   DESIGNS 


made  slightly  longer.  The  casting  should  be  finished,  all  holes  drilled 
and  secured  to  the  base,  as  shown  in  Fig.  199.  A  brass  button  should 
be  turned  up  and  a  slot  cut  therein  to  receive  the  end  of  a  piece  of  1-16 
in.  sheet  brass,  7-16  in.  wide,  which  should  be  soldered  into  this  slot. 
A  3-16  in.  hole  is  drilled  through'  the  end  of  this  strip,  i^  in.  from  the 


American  EUftrMm 


FIG.    iq8. 


face  of  the  casting,  to  the  top  of  which  this  button  is  screwed.  A  milled 
head  screw,  3-16  in.  in  diameter  and  I  5-16  in.  long,  is  provided  with  two 
milled  nuts,  and  has  a  small  hook,  made  from  No.  20  hard  brass  wire, 
soldered  to  its  end.  This  screw  is  passed  through  the  hole  in  the  brass 
strip;  screw  and  nuts  are  shown  in  Figs.  197  and  198. 


D'ARSONVAL    GALVANOMETER  177 

The  cylinder  shown  in  Figs.  197,  198  and  199  is  turned  from  soft 
iron,  and  is  supported  by  a  brass  arm  from  the  standard.  Its  position 
is  plainly  shown  in  Fig.  198.  A  spring  of  1-32  in.  brass,  5-16  in.  wide, 
is  supported  from  a  small  brass  pillar  in  the  position  shown  in  Fig.  198. 
A  small  milled-head  screw  passes  through  this  spring  and  into  a  threaded 


FIG.    199. 


plate  set  into  the  base.  This  screw  serves  to  adjust  the  tension  on  the 
spring  to  which  the  lower  suspension  is  attached.  A  small  hook,  made 
from  No.  18  or  20  brass  wire,  is  soldered  to  the  end  of  the  spring  in 
such  a  manner  that  it  will  be  exactly  under  the  center  of  the  iron  cylin- 
der. One  of  the  leveling-screws  is  shown  in  Fig.  200  and  their  positions 
are  indicated  in  Figs.  197  and  199. 

Two  small  binding  posts  are  fastened  on  the  extreme  edge  of  the 


178 


ELECTRICAL  DESIGNS 


base  so  as  to  be  outside  of  the  glass  globe,  which  must  cover  the  com- 
pleted instrument  and  rest  upon  the  base.  These  posts  are  to  be  con- 
nected beneath  the  base,  one  to  the  small  pillar  on  the  front  of  the  in- 
struments and  the  other  to  the  standard. 

To  wind  the  coil  a  form  is  necessary;  this  is  made  from  a  piece 
of  brass  K  in.  by  I  3-6  in.  by  il/2  in.  with  a  plate  \y2  in.  by  I  15-16 
in.  fastened  by  screws,  to  each  side.  The  corners,  over  which  the  wire 
bends,  should  be  slightly  rounded.  The  wire  to  be  used  should  be  silk- 
covered  and  should  not  be  larger  than  No.  36  B.  &  S.,  and  the  finer  it  is 
the  more  sensitive  will  be  the  instrument.  Wind  it  carefully  and  in  even 
layers,  using  no  paper  or  other  insulation  aside  from  that  on  the  wire. 
The  completed  coil  should  be  I  7-16  in.  wide,  but  the  exact  length  is 
immaterial.  Place  the  form  with  the  wire  still  on  it  in  an  oven  and  heat 
until  as  hot  as  the  hand  can  bear;  then  with  a  clean  soldering  copper, 
drop  on  paraffine  until  the  wire  is  completely  saturated  with  it.  After  it 
has  cooled,  carefully  remove  the  coil  from  the  form  and  trim  off  the 
superfluous  paraffine. 


Jfi 


Cut  from  very  thin  copper,  such  as  a  leaf  from  a  dynamo  brush,  two 
plates  34  in.  by  5-8  in.  and  to  the  center  of  one  solder  a  small  hook 
made  from  No.  20  or  22  hard  brass  wire ;  to  the  other  one  solder  a  similar 
hook,  but  with  a  shank  7-16  in.  long.  With  a  fine  silk  thread  bind  one 
of  these  plates  to  each  end  of  the  coil,  being  careful  to  place  a  thin  piece 
of  mica  between  the  plate  and  the  coil,  and  to  have  the  hooks  exactly 
in  the  center.  To  each  plate  solder  one  terminal  of  the  coil  and  shellac 
all  but  the  hooks. 

A  mirror  3-8  in.  in  diameter  is  now  to  be  cemented  to  the  shank 
of  the  longer  hook  by  means  of  a  little  thick  shellac  varnish.  This 
mirror  may  be  made  from  a  microscope  cover  glass,  and  silvered  by  the 
following  formula:'  Take  100  parts  by  volume  of  a  10  per  cent,  solution 
of  nitrate  of  silver  and  add,  drop  by  drop,  a  quantity  of  ammonia  just 
sufficient  to  dissolve  the  precipitate  formed.  Make  up  the  volume  to 
ten  times  the  amount  by  adding  distilled  water.  Dilute  a  40  per  cent, 
solution  of  formaldehyde  to  a  I  per  cent,  solution.  Dip  the  glass,  pre- 
.viously  cleaned  with  chamois,  in  a  mixture  of  two  parts  of  silver  solution 


D'ARSONVAL    GALVANOMETER  179 

to  one  of  formaldehyde.  After  ten  to  fifteen  minutes  wash  in  running 
water  and  varnish  the  back.  The  silver  will  adhere  to  both  sides'  and 
must  be  removed  from  the  face. 

The  coil,  with  its  mirror,  is  now  to  be  suspended  between  the  hook 
on  the  screw  at  the  top  and  the  one  on  the  spring  at  the  bottom.  The 
suspension  is  a  very  fine  phosphor  bronze  wire  or  strip,  and  can  be  best 
obtained  from  the  makers  of  such  instruments.  For  the  suspensions 
take  two  pieces  of  the  proper  length  and  form  a  small  loop  in  each  end 
of  each.  These  loops  must  fit  snugly  over  the  hooks.  Suspend  the  coil 
by  these,  having  the  mirror  at  the  top.  Adjust  the  instrument  so  that 
the  suspensions  are  taut  with  some  little  strain  on  them  from  the  lower 
spring.  See  that  the  coil  swings  freely  between  the  magnet  limbs  and 
the  iron  cylinder  and  is  parallel  with  the  front  of  the  magnet. 

The  instrument  is  then  complete,  but  should  be  provided  with  a 
reading  telescope  or  a  scale  and  lamp,  which  is  not  quite  as  convenient, 
but  is  simpler.  It  consists  of  a  board  2  ft.  long  attached  to  a  base  and 
carrying  a  scale  at  about  the  same  height  as  the  mirror  on  the  galvano- 
meter. Just  below  the  center  of  the  scale  is  a  3-4  in.  hole  with  a  fine 
wire  stretched  perpendicularly  across  it.  A  lamp  is  placed  with  its  flame 
opposite  the  hole  and  behind  it  and,  by  means  of  a  suitable  lens,  the 
image  of  this  wire  is  thrown  on  the  mirror  and  reflected  back  to  the 
scale,  thus  acting  as  a  pointer.  An  ordinary  magnifying  glass  will  answer 
in  the  absence  of  a  better  lens.  The  scale  should  be  about  2  ft.  from  the 
galvanometer  and  the  lens  should  be  between  the  two,  rather  nearer  the 
scale.  The  exact  positions  must  be  left  to  experiment  in  each  individual 
case. 


CHAPTER  XXI. 


SENSITIVE  MIRROR  GALVANOMETER. 


The  instrument  described  herewith  is  intended  to  obviate  almost  en- 
tirely the  necessity  for  skilful  manipulations,  upon  the  principle  which 
pays  so  well  in  the  machine  shop,  viz.,  that  the  whole  be  so  designed  in  its 
several  parts,  that  the  machine  work  shall  be  reduced  to  a  minimum,  or 
even  dispensed  with  altogether,  save  a  little  drilling,  etc. 

The  above  scheme  has  been  adopted  in  making  the  galvanometer, 
which,  after  having  been  turned  out  "with  jack-knife  and  pliers,"  will 


FIG.    2OI. — SENSITIVE    "HOME-HADE"    GALVANOMETER. 

give  results  closely  approaching  those  received  from  a  more  elaborate 
and  costly  instrument.  Fig.  201  give*  a  view  of  the  instrument  com- 
plete. It  consists  of  five  parts — the  lamp,  the  screen,  the  lens,  the  coils 
and  the  needles. 

For  the  lamp,  a  bicycle  lamp  leaves  nothing  to  be  desired,  though  a 
common  kerosene  hand  lamp,  as  shown  in  the  engraving,  answers  every 
purpose.  The  vertical  board  is  as  high  as  the  lamp,  and  the  scale  is 


SENSITIVE  MIRROR   GALVANOMETER 


181 


attached  to  the  top  edge  of  the  board.  The  scale  may  be  an  ordinary 
yardstick,  or  ruler  fastened  to  the  board,  or  it  may  be  a  strip  of  paper 
ruled  to  millimeters,  and  shellacked  to  the  board. 

The  tin  shade  is  simply  to  cut  off  some  of  the  light  which  otherwise 
would  be  reflected  over  the  top  of  the  scale,  and  dim  the  bar  of  light. 
A  clean,  sharp  slit  may  be  made  by  cutting  a  somewhat  large  hole  in  the 
board,  and  covering  it  with  a  bit  of  cardboard  or  brass,  in  which  a  slit  of 
the  size  found  by  experience  to  be  best  has  been  cut. 

The  lens  may  be  an  ordinary  reading  glass,  or  it  may  be  one  of  the 
cheap  lenses  to  be  obtained  in  almost  any  shop  for  a  few  cents.  Almost 
any  form  of  lens  can  be  made  to  answer,  but  preferably  it  should  be  a 
double  convex  of  very  long  focus — 16  ins.  to  18  ins.  If  a  reading  glass  is 


FIG.   202. — METHOD  OF  MOUNTING  PLAIN 
LENS. 


FIG. 


203. — CAP  FOR  TOP  OF  GLASS 
CHIMNEY. 


used,  it  may  be  mounted  by  placing  the  handle  through  a  hole  in  the  base- 
board as  shown.  If  a  plain  lens  is  to  be  used,  a  cheap  mount  is  shown  by 
Fig.  202.  A  bit  of  board  is  cut  out  as  shown,  and  the  hole  through  it  is 
just  a  trifle  smaller  than  the  lens.  A  narrow  V-shaped  groove  is  then  cut 
around  the  center  of  the  inside  of  the  hole,  and  a  saw  kerf  run  into  the 
board  as  shown.  This  allows  the  lens  to  be  pressed  into  the  groove,  and 
the  spring  of  the  wood  holds  it  there. 

The  six  leveling  screws  are  common  brass  wood-screws,  4  ins.  long, 
about  34  in.  in  diameter,  with  the  top  of  the  head  filed  off  flat.  The 
edges  of  the  disc  thus  formed  may  be  milled  in  pretty  good  shape  by  roll- 
ing the  edge  of  the  head  under  a  single-cut  file  of  the  required  degree  of 
fineness.  Place  the  screw  on  a  hardwood  board,  or  better  yet,  on  a  slieet 


1 82  ELECTRICAL  DESIGNS 

of  lead,  and  by  rolling  under  a  file,  the  milling  can  be  quickly  done.    By 
all  means  use  a  lathe  if  you  have  one.  in  preference  to  the  file  method. 

The  third  member  is  built  on  a  bit  of  board  cut  about  8  ins.  on  a  side 
of  triangular  shape,  as  shown.  Three  leveling  screws  are  let  in,  and  two 
binding  posts  are  placed  in  connection  with  the  coil.  These  posts  are 
shown  in  the  engraving.  A  common  medium  sized  lamp  chimney  is  pro- 
cured and  fitted  to  a  circular  piece  of  wood  %  m-  thick.  The  wood  is 
screwed  to  the  base,  and  the  coils  are  fastened  to  the  wocd;  the  mirror 
must  be  placed  one  meter  (39.37  ins.)  from  the  scale. 

Another  circular  piece  of  wood  is  fitted  to  the  top  of  the  chimney, 
as  shown  in  Fig.  201.  A  detail  plan  and  section  of  this  piece  is  shown  by 
Fig.  203.  It  is  bored  out  to  fit  on  the  chimney,  and  a  3^  inch  hole  is 
bored  in  the  center,  completely  through  the  wood.  A  wire  with  a  sort 
of  thumb-head  is  bored  into  the  wooden  cap  so  as  to  pass  through  the 
center  of  the  ^  in.  hole.  A  bit  of  cardboard  is  glued  into  the  bottom 
of  the  large  hole,  and  a  pin-hole  punched  through  the  exact  center,  per- 
mits the  suspension  fibre  of  the  needle  system  to  be  carried  to  the  wire 
and  wound  up  by  turning  the  thumb-head  above  described. 

The  coils  may  be  made  according  to  the  work  to  be  done ;  the  writer 
has  three  sets  of  coils  with  his  own  instrument,  two  in  each  set,  and  uses 
whichever  set  the  character  of  the  work  requires.  The  first  is  made  of 
about  50  ft.  of  single-silk-covered  copper  magnet  wire,  the  size  being  No. 
19,  B.  &  S.  gauge.  The  second  set  of  two  coils  is  wound  of  No.  33  or  No. 
34  wire.  The  third  coil  is  wound  with  No.  36  wire.  Nearly  y±  lb.  was 
put  on  the  two  coils,  and  the  combined  resistance  of  the  complete  coils  is 
about  1,000  ohms — 500  ohms  each. 

A  form  for  winding  the  coils  is  shown  by  Fig.  204.  It  is  made  of 
wood,  held  together  with  two  screws.  A  couple  of  binding  wires  are  laid 
in  before  the  coil  is  wound.  About  six  layers  of  the  wire  above  men- 
tioned ca«*be  made  out  of  one-half  the  25  ft.  mentioned.  Two  of  these 
coils  are  used,  connected  in  series  and  to  the  binding  posts.  After  wind- 
ing, the  binding  wires  are  fastened,  the  coil  is  drenched  with  shellac  and 
placed  in  the  cook-stove  oven  for  an  hour.  The  core  is  then  removed, 
additional  binding  placed  on  the  coil  if  found  necessary,  and  again  baked 
at  low  heat  for  two  or  three  hours.  This  holds  the  coil  permanently. 
Two  coils  are  to  be  used,  and  the  needle  system  suspended  between  the 
coils,  which  are  placed  3-8  inches  apart. 

For  the  needles  with  the  low  resistance  coils  the  writer  used  a  com- 
mon sewing  needle.  The  temper  was  drawn,  the  eye  and  point  filed  off, 
leaving  a  bit  of  wore  ij4  ms-  l°n£T-  A  nick  was  filed  in  the  center,  then 
the  needle  was  hardened  and  magnetized,  and  broken  througfr  the  nick, 
thus  giving  two  needles  magnetized  pretty  near  alike.  A  piece  of  card- 


SENSITIVE  MIRROR   GALVANOMETER 


183 


board  2  ins.  X  j^  in.  was  pierced,  and  the  needle  stuck  through  it,  as  in 
Fig.  205,  and  held  by  a  drop  of  hot  sealing  wax. 

A  bit  of  mirror,  m,  was  waxed  to  the  top  of  the  cardboard,  and  the 
suspension  fibre  fastened  between  the  mirror  and  the  cardboard,  as 
shown.  The  upper  end  of  the  fibre  is  carried  to  the  cap  on  top  of  the 
chimney,  attached  to  the  thumb-head  wire,  and  wound  up  until  the  lower 
needle  hangs  in  the  middle  of  the  coil,  and  the  upper  needle  clears  the  top 
of  the  coil  about  }4  m-  The  instrument  is  now  ready  for  setting  up  and 
adjusting  in  the  usual  manner. 


FIG.   204. — FORM   FOR  WINDING  COILS. 


FIG.    205. — NEEDLE  SUSPENSION, 


The  sse^nd  set  of  needles  is  made  in  the  same  manner,  except  that 
pieces  of  fine  watch  spring  less  than  %  in.  wide,  may  be  used  preferably, 
three  pieces  being  placed  together  with  a  single  thickness  of  paper  be- 
for  each  needle.  The  pieces  should  be  file-marked,  hardened,  magnetized 
and  broken  in  pieces,  the  same  as  the  needles. 

Finding  that  the  light  needles  and  the  low  resistance  coils  gave  an 
instrument  readily  affected  by  thermal  currents,  the  writer  made  the  third 
.set  of  needles  of  steel  tape  about  ^  in.  wide,  and  used  five  pieces  in  each 
needle,  separating  each  with  a  paper.  All  the  needles  in  the  three  sys- 
tems were  $i  in.  long.  The  third  set  was  rather  heavy,  but  in  con- 


ELECTRICAL  DESIGNS 

nection  with  the  i,oooohm  coils  proved  very  sensitive,  although  slow- 
moving. 

Different  effects  were  secured  by  using  either  set  of  the  needles  with 
the  other  coils,  making  six  possible  combinations.  Where  extreme  sensi- 
tiveness is  not  required,  the  writer  found  it  desirable  to  use  a  directing 
magnet,  and  not  depend  upon  the  torsion  of  the  suspension,  or  over- 
strength  of  one  of  the  needles,  to  return  the  beam  of  light  to  zero. 

With  1,000  ohms  in  each  arm  of  the  bridge,  and  6  volts  from  the 
battery,  a  considerable  deflection  is  obtained  by  changing  R  a  single 
ohm,  and  with  the  bridge  adjusted  at  1,000  to  I  at  a  and  £,  the  galvano- 
meter readily  deflects  beyond  the  capacity  of  the  bridge,  which  is  .00 1 
ohm,  with  1,000  ohms  galvanometer  resistance. 

For  the  suspension  in  this  instrument  the  reader  may  use  a  hair,  quite 
fine,  say  about  .002  in.  in  diameter.  From  the  needles  to  the  point  of  sus- 
pension there  should  be  about  8  in.  of  effective  hair.  Just  how  much 
better  the  instrument  would  be  with  a  raw  silk  fibre  the  writer  has  no 
means  of  knowing  at  present,  but  it  was  as  delicate  as  will  be  required 
for  any  ordinary  work.  The  "efficiency"  of  the  low-resistence  instrument 
is  rather  greater  than  that  of  the  high-resistance  form,  while  the  "figure 
of  merit"  is  greater  the  more  turns  of  wire  are  placed  on.  For  meas- 
uring very  low  resistances,  the  low-resistance  coils  will  give  perhaps  the 
best  results. 


CHAPTER  XXII. 


A  THOMSON  ASTATIC  GALVANOMETER, 


While  the  Thomson  astatic  galvanometer  has  been  more  or  less  su- 
perseded by  the  D'Arsonval  instrument,  it  is,  nevertheless,  an  excellent 
instrument  for  the  detection  of  feeble  currents  where  great  sensitiveness 
is  required.  It  is,  therefore,  used  for  nul  or  zero  methods,  such  as  measur- 
ing resistance  by  the  Wheatstone  bridge;  Rayleigh's  and  Bosscha's 


Am.Elec. 


FIG.    206. — FRAME   OF   GALVANOMETER. 


methods  of  comparing  the  electro-motive  force  of  primary  cells ;  Thom- 
son's method  of  comparing  the  electrostatic  capacities  of  two  con- 
densers, etc.  It  requires,  perhaps,  more  skill  and  patience  on  the  part 
of  the  user  than  the  D'Arsonval,  and  cannot,  like  the  latter,  be  so  readily 
used  in  the  neighborhood  of  dynamos  or  wires  carrying  strong  and  vari- 
able electric  currents. 


186 


ELECTRICAL   DESIGNS 


The  galvanometer  about  to  be  described  has  proved  a  very  useful 
instrument  for  laboratory  work  and  may  be  made  very  sensitive  if  de- 
sired. 

The  woodwork,  as  shown  in  Figs.  206  and  207,  is  made  up  of  four 
pieces.  Into  the  base  is  mortised  and  glued  the  upright  portion,  con- 
sisting of  a  thin  central  sheet  or  diaphragm,  3-32  in.  thick,  glued  between 
the  two  cup-carrying  supports.  This  thin  partition  holds  the  two  coils 
apart  and  is  cut  away,  as  indicated  in  Figs.  206  and  208,  to  give  sufficient 
room  for  the  suspended  system.  A  hole,  7-16  in.  in  diameter  and  2  ins. 
deep,  should  be  drilled  from  the  top,  and  another  hole  in  the  front,  3-4 
in.  in  diameter,  as  shown  in  Figs.  206  and  208. 

This  front  opening  is  to  be.  covered  with  a  clear  piece  of  perfectly 
plane  glass,  ground  square  or  round,  as  shown  in  Fig.  215.  To  make  this 


FIG.    2O7. — COIL  CUPS. 

glass  fit  air-tight  and  yet  not  break  when  the  screws  are  tightened,  put 
a  washer  of  rubber,  felt,  or  chamois  skin  under  the  edge.  Fig.  208 
is  a  half-tone  picture  of  a  similar  instrument.  After  the  two  main  binding 
posts,  three  leveling  screws,  two  spirals  of  No.  26  silk-covered  copper 
-wire,  soldered  to  the  under  nuts  or  washers  of  the  binding  posts,  and 
two  brass  rods  for  holding  the  coil  cups,  have  been  put  in  place,  set  the 
"binding  posts  well  apart  and  near  the  edge  of  the  base  to  allow  plenty  of 
Toom  for  subsequently  putting  in  place  and  removing  the  front  coil-cup, 
pig.  207  is  a  scale  drawing  of  the  wooden  cups  for  holding  the  coils. 
These  cups  should  fit  snugly  into  the  uprights  in  order  to  be  as  air-tight 
&s  possible. 

Well  seasoned,  hard  maple  is  a  very  good  wood  to  use  and  polishes 


THOMSON  ASTATIC  GALVANOMETER 


187 


nicely.  The  inside  surfaces  of  the  woodwork  should  be  treated  to  a  coat 
of  shellac.  To  give  the  outside  a  fine  polish  proceed  as  follows :  After 
making  the  outside  surface  as  smooth  as  possible  with  very  fine  sand- 
paper, coat  it  with  hard  oil.  When  perfectly  dry,  rub  it  down  well  with 
powdered  pumice  stone,  using  a  soft  rag  and  boiled  linseed  oil  thinned 
with  kerosene.  Repeat  this  several  times  and  the  last  time  or  two  use 
powdered  rotten  stone  in  place  of  the  pumice  stone. 

Fig.  209  shows  one  of  the  leveling  screws  and  Fig.  213  the  tapped 
brass  rod  and  nuts  for  holding  the  cups  in  place.  There  are  two  of  these 
brass  rods,  threaded  their  entire  length,  eight  brass  washers,  four  small 
square  brass  nuts  and  four  larger  ones.  Two  of  the  latter  are  shown  oa 


FIG.    208. 


FIG.    215, 


FIG.    2l6. 


the  front  of  the  finished  galvanometer  (Fig.  216).  Fig.  207  shows  how 
these  rods  are  fastened  in  place  by  the  small  square  nuts,  one  nut  and 
washer  on  each  side  of  the  thin  partition. 

Fig.  210  shows  a  form  on  which  the  coils  are  wound.  The  one  from 
which  this  drawing  was  made  was  constructed  of  iron  because  a  large 
number  of  coils  were  wound  upon  it,  but,  where  only  two  coils,  as  in  the 
present  case,  are  to  be  wound,  it  could  be  made  of  some  hard  wood  by 
slight  modification.  Before  winding  the  coil  cover  that  part  of  the  form 
which  is  to  be  filled  up  with  wire,  with  two  or  three  thicknesses  of  par- 
affined paper  to  facilitate  the  removal  of -the  coil  when  finished.  The 
wire  coil  should  fill  the  form  to  within  %  in.  of  the  edge.  To  make  a  coil 
with  a  resistance  of  200  ohms,  making  a  4OO-ohm  instrument  when  the 


iS8  ELECTRICAL  DESIGXS 

two  coils  are  connected  in  series,  will  require  about  13  ozs.  of  No.  30 
B.  &  S.  double-silk-covered  copper  wire.  By  connecting  the  two  coils 
in  parallel,  instead  of  in  series,  the  galvanometer  will  have  100  ohms 
resistance.  In  winding  these  coils  it  is  well  to  first  wind  on  a  single  layer 
of  No.  26  wire,  to  which  the  No.  30  is  soldered,  and  to  terminate  the 
winding  with  a  layer  of  No.  26.  This  protects  the  finer  wire  and  furnishes 
a  more  suitable  wire  for  making  soldered  connections  to  the  binding  post 
washers.  In  soldering  connections  use  resin  and  not  soldering  fluid, 
as  the  latter,  unless  completely  neutralized  and  dried  off,  may  later  cor- 
rode the  metal  and  cause  trouble. 

The  coil,  when  wound,  should  be  well  treated  with  shellac  and  al- 
lowed to  dry  before  attempting  to  remove  it  from  the  form.  To  do  this, 
it  is  necessary  to  heat  the  form  until  the  paraffined  paper  softens  enough 
to  allow  the  removal  of  the  coil  from  the  form.  The  shellac  should  hold 
the  coil  in  shape.  The  ends  of  the  coil  are  soldered  to  washers  which  are 
firmly  fastened  in  counter-sunk  holes  in  the  inside  of  the  wooden  cup 
by  the  binding  post  screws.  After  placing  the  coil  in  the  center  of  the 
cup  pour  a  mixture  of  melted  paraffine  and  resin  into  the  cup,  around  the 
coil,  to  hold  it  there.  Paraffine  alone  is  too  soft  in  warm  weather  to  sus- 
tain the  coil  in  place.  Fig.  211  is  a  horizontal  cross  section  of  cup  and 
coil  complete.  Fig.  215  gives  an  inside  and  outside  view  of  the  same. 
When  the  coils  are  in  place  the  hollows  at  the  centers  of  the  coils  should 
come  exactly  opposite  each  other  and  just  above  the  bottom  of  the  cut- 
away portion  of  the  separating  wooden  partition  in  the  middle  of  the 
upright  supports,  and  the  surface  of  both  coils  should  press  against  this 
separating  partition. 

A  more  sensitive  instrument,  having  the  same  resistance,  can  be 
made  by  winding  the  two  coils  as  follows :  Over  a  single  layer  of  No.  26, 
wind  136  ohms  of  No.  36,  heaping  up  the  wire  slightly  toward  the  face 
of  the  coil  which  is  to  be  nearest  the  needle — that  is,  on  the  side  of  the 
form  where  the  conical  spindle  has  the  smallest  diameter.  Over  this 
wind  52  ohms  of  No.  31,  and  finally,  put  on  12  ohms  of  No.  26. 

The  two  coils  are  to  be  connected  either  in  series  or  parallel,  by 
spiral  wires  running  through  and  under  the  wooden  base,  so  that  the 
current,  flowing  through  both,  will  tend  to  deflect  the  needle  system  in  one 
and  the  same  direction ;  that  is,  one  coil  must  not  oppose  the  other  in  its 
action  upon  the  magnetized  needle.  The  ends  of.  these  spiral  wires 
should  be  soldered  to  washers  under  the  nuts  of  the  main  binding  posts. 
Two  main  binding  posts,  similar  to  Fig.  217  and  four  smaller  ones,  for 
the  coil  cups,  similar  to  Fig.  218  will  be  needed. 

Fig.  212  shows  two  brass  pieces  which  are  connected  together  by 
a  srtout  piece  of  glass  tubing,  6  ins.  in  length.  The  outside  diameter  of 


THOMSON  ASTATIC  GALVANOMETER 


189 


FIG.    209. 


FIG.    211. 


FIG.    212. — DETAILS   OF  SUSPENSION  TUBE. 


FIG.   213 


— * 

*fi. 


FIG.  217.     FIG. 


FIG.   2Z4. 


190  ELECTRICAL  DESIGNS 

this  glass  tube  should  be  enough  smaller  than  V*  in.  (the  diameter  of  the 
hole  into  which  it  is  to  go)  to  allow  a  piece  of  chamois  skin  to  be 
wrapped  once  around  the  tube  and  thus  make  the  two  ends  fit  snugly  into 
the  two  brass  pieces,  A  and  B.  In  order  that  it  may  not  turn,  the  bottom 
one  is  to  be  firmly  secured  with  shellac  or  LePage's  glue  on  both  sides 
of  the  chamois  skin ;  but  the  top  piece  of  the  skin  is  glued  on  only  one 
side  to  allow  the  top  brass  piece  to  be  revolved  around  the  glass  tube  so 
that  the  torsion  may  be  removed  from  the  suspending  fibre,  whenever 
necessary,  in  adjusting  the  instrument  to  give  zero  reading.  By  using  a 
longer  glass  tube  and,  consequently,  making  the  suspending  silk  fibre 
longer,  the  sensitiveness  of  the  galvanometer  may  be  increased. 

The  bottom  brass  piece,  A  (Fig.  212),  is  fastened  to  the  top  of  the 
wooden  framework  by  four  brass  screws;  but,  between  the  wood  and 
metal,  a  piece  of  felt  is  interposed.  This  is  in  the  form  of  a  circular  disc 
of  i  3-8  ins.  external  diameter  and  cut  away  at  the  center  to  allow  pas- 
sage for  the  suspending  fibre.  This  felt  serves  two  purposes :  First,  it 
prevents  air  currents  passing  through  and  disturbing  the  action  of  the 
needle  system;  secondly,  it  affords  a  means  of  adjusting  the  suspension 
tube  into  perfect  alignment  with  the  rest  of  the  instrument  by  tighten- 
ing the  proper  screws. 

It  is  perhaps  needless  to  remark  that  no  iron  in  any  form,  except  for 
the  needles,  must  be  used  in  the  construction  of  this  form  of  galvano- 
meter. 

The  astatic  needle  systeni  is  shown  in  Fig.  214.  It  consists  of  a  very 
thin,  rectangular  piece  of  mica,  ^  in.  X  i?4  ms-> to  which  are  fastened,  by 
shellac,  a  plane  glass  mirror  from  y2  in.  to  5-8  in.  in  diameter,  the  upper 
set  of  steel  needles,  and  a  thin  glass  fibre  about  1-50  in.  in  diameter. 
The  object  of  the  mica  vane  is  to  dampen  the  vibrations  and  help  bring 
the  needle  system  to  rest  and,  for  this  purpose,  it  should  be  as  large  as 
the  space  in  which  it  swings  will  allow.  The  space  which  contains  the 
needle  system  and  the  silk  suspending  fibre  should  be  made  as  air- 
tight as  possible  to  prevent  external  gusts  of  wind  or  currents  of  air 
from  entering  and  causing  the  needle  system  to  vibrate  and  shake.  When 
finished  and  set  up  ready  for  use,  the  mirror  should  come  opposite  the 
glass  covered  hole  in  the  front  of  the  instrument.  It  is  difficult  to  make 
a  good  mirror  and  it  is  more  satisfactory  to  purchase  one  from  an  electrical 
instrument  maker.  However,  if  the  reader  desires  to  make  one  he  will 
find  the  necessary  directions  on  page  178. 

The  glass  fibre  used  to  connect  the  lower  set  of  needles  to  the  mica 
vane  on  which  the  upper  set  of  needles  is  fastened,  must  be  perfectly 
straight  and  very  light.  By  a  little  practice  a  good  one  can  be  made 
from  a  small  glass  tube  by  heating  it  over  a  fish-tailed  gas  burner  and 


THOMSON  ASTATIC  GALVANOMETER 


191 


drawing  it  out.  Make  a  number  of  these  and  select  the  best  one.  To 
the  top  of  this  secure,  by  shellac,  a  minute  hook  made  of  No.  36  or  No. 
38  bare  copper  wire,  and  to  the  lower  end  glue  a  very  thin  circular  piece 
of  mica  upon  which  is  glued  one  set  of  steel  needles.  The  two  pieces 
of  mica  must  be  in  the  same  plane.  This  needle  system  should  be  as  light 
as  possible  (not  over  ten  grains),  to  make  its  moment  of  inertia  small. 
Other  things  being  equal,  the  smaller  this  moment  of  inertia  the  quicker 
it  will  come  to  rest. 

The  steel  needles  may  be  made  from  steel  piano  wire  about  1-40  to 
1-50  in.  in  diameter,  or  No.  8  guitar  string.  The  temper  should  first  be 
drawn  and  the  wire  straightened  and  cut  into  convenient  lengths — about 


Battery 


FIG.   219. — METHOD  OF  MAGNETIZING  NEEDLES. 

6  ins.  long.  These  pieces  must  then  be  tempered  glass  hard.  This 
should  be  carefully  done,  as  the  entire  piece  should  be  of  the  same  hard- 
ness. From  this  wire  cut  or  break  off  twelve  pieces  of  the  proper  length 
(as  indicated  in  Fig.  214)  for  the  needles  and  secure  them  in  place  with 
shellac,  six  above  and  six  below,  being  careful  to  have  a  thin  air  space, 
about  1-50  in.  between  each  needle,  so  that  they  will  not  touch  each  other. 
The  best  way  to  magnetize  the  needles  and  obtain  the  astatic  system — 
that  is  one  set  magnetized  equally  but  oppositely  to  the  other — is  to  mag- 
netize the  set  by  an  apparatus  shown  in  Fig.  219.  A  and  B  are  soft- 
iron  rods,  7-16  in.  in  diameter.  C,  D,  E  and  F  are  soft-iron  pole  pieces 
between  which  the  needles  to  be  magnetized  are  placed,  as  shown.  G 
and  H  are  two  coils,  each  consisting  of  350  turns  of  No.  18  B.  &  S. 
copper  wire.  The  coils  must  be  connected  in  series  so  that  the  magnetic 


192  ELECTRICAL   DESIGNS 

potential  of  each  will  produce  a  magnetic  flux  and  poles  as  indi- 
cated in  the  figure.  The  trough  is  merely  a  variable  liquid  resistance 
containing  some  liquid,  such  as  salt  water  or  a  solution  of  washing  soda, 
and  two  metal  plates  or  electrodes,  one  of  which  can  be  moved  from  one 
end  to  the  other,  so  that  the  current  may  be  varied  from  o  to  about  5  am- 
peres. Any  other  convenient  variable  resistance  which  will  do  this  may, 
of  course,  be  used. 

After  making  connections  and  adjusting  the  strength  of  the  solution 
so  that  the  proper  current  can  be  obtained,  place  the  needles  between  the 
pole  pieces,  as  in  Fig.  219,  and  put  on  the  full  current  of  5  amperes  and 
then  gradually  and  slowly  reduce  the  current  to  zero.  Repeating  this 
process  several  times  should  magnetize  the  needle  sufficiently,  and  if  the 
steel  were  properly  hardened,  it  may  never  need  remagnetizing. 


FIG.    22O. — TELESCOPE  AND   SCALE. 


For  suspending  the  needle  system,  get  a  single  fibre  of  unspun  silk  at 
least  10  ins.  long.  To  remove  all  initial  torsion,  hang  this  up  for  a  day  or 
so,  having  fastened  to  its  lower  end  a  small  weight  of  non-magnetizable 
material,  such  as  a  brass  screw.  Pass  this  silk  fibre  through  the  1-32  in. 
hole  in  B,  Fig.  212,  and  secure  one  end  to  C,  by  tying  it  through  the  1-32 
in.  hole  in  that  piece.  One  or  both  of  the  coil  cups  being  removed,  work 
the  free  end  of  the  silk  fibre  down  through  the  glass  tube.  Fasten  this 
free  end  to  the  small  hook  on  the  upper  end  of  the  needle  system  with 
shellac.  When,  the  shellac  is  dry,,  wind  up  the  fibre  on  C,  until  the  lower 
needles  hang  in  the  center  of  the  coils.  Then  level  the  galvanometer 
until  the  lower  needles  hang  midway  between  the  two  coils  and  turn 
perfectly  free.  With  both  coil  cups  in  place  and  the  galvanometer  prop- 
erly leveled,  the  needle  system  should  still  vibrate  free  and  smooth.  It 
will  depend  upon  the  resultant  polarity  of  the  astatic  system  whether  the 


THOMSON-  ASTATIC  GALVANOMETER  193 

instrument  should  be  set  facing-  East  cr  West.  This  can  best  be  deter- 
mined by  trial. 

If  a  plane  mirror  is  used,  a  telescope  and  scale  for  observing  the  deflec- 
tion of  the  needle  system  is  more  convenient  than  a  lamp,  scale  and  lens. 
A  telescope,  suitable  for  this  purpose,  can  be  purchased  for  $1.80  or  less, 
and  a  one-half  meter  scale  on  cardboard  can  be  obtained  for  twenty-five 
cr  fifty  cents.  If  a  terrestrial  telescope  is  used,  it  may  be  greatly  im- 
proved, for  this  purpose,  by  removing  the  set  of  rectifying  lenses,  in 
which  case  the  figures  on  the  scale  must  be  reversed  and  inverted.  Fig. 
220  shows  a  suitable  stand  for  this  purpose,  which  may  be  easily  made. 
It  is  customary  to  place  the  telescope  and  scale  just  one  meter  from  the 
mirror.  The  rear  support  for  the  telescope  and  the  board  upon  which 
the  paper  scale  is  fastened  are  arranged  to  slide  up  and  down  for  adjust- 
ing. The  scale  should  be  as  much  below  the  level  of  the  mirror  as  the 
object  glass  of  the  telescope  is  above  it. 

Quite  a  number  of  galvanometers  of  the  above  type  with  the  telescope 
and  scale  have  been  in  regular  use  in  the  electrical  laboratory  at  Lehi^h 
University  for  the  past  four  or  five  years.  There  are  also  several  four- 
coil  galvanometers  of  a  similar  design,  whose  resistances  run  up  as 
high  as  5,000  ohms,  but  for  general  use  the  two-coil  instruments  are  sen- 
sitive enough.  Considerable  cf  the  work  on  these  galvanometers  was 
done  by  student's  taking  the  electrical  course  during  their  third  year. 


CHAPTER  XXIII. 


A   CHEAP  TESTING  SET. 


The  Wheatstone  bridge  is  one  of  the  most  valuable  instruments  that 
can  be  devised,  not  only  for  its  primary  office  of  measuring  resistances, 
but  for  testing  faults  of  any  kind.  Most  beginners  are  not  aware  that  a 
convenient  substitute  can  be  made  that  will  answer  in  many  cases,  and  the 
expense  of  which  need  not  exceed  two  or  three  dollars.  Following  a  de- 
scription of  such  an  easily  made  apparatus  is  given,,  and  it  may  be  of  in- 
terest to  those  anxious  to  possess  a  testing  set  and  who  do  not  feel  that 
they  can  afford  the  large  sum  usually  asked  for  such  an  outfit. 

Select  a  board  of  some  dry  wood  and  shape  it  nicely  to  the  dimensions 
of  42  ins.  long  by  8  ins.  wide.  Procure  some  flat  copper  rod  which  is  at. 
least  y2  by  y\  of- an  inch  in  sectional  area.  Of  this  there  should  be  one 
continuous  bar  36  ins.  long,  and  two  shorter  pieces  about  3  ins.  long. 
Then  secure  a  good  straight  piece  of  German  silver  wire,  about  No.  14 
B.  &  S.  gauge.  Go  over  this  with  a  micrometer,  testing  it  at  every  inch 
in  its  length,  and  be  sure  that  the  piece  selected  is  of  uniform  diameter 
and  has  no  nicks  or  marks  of  any  kind. 

With  a  suitable  size  of  drill  bore  holes  34  of  an  inch  from  the  ends  ci 
the  short  pieces  of  copper  rod,  and  J/£  of  an  inch  from  the  flat  sides,  as 
shown  in  Fig.  221.  The  wire  should  now  be  soldered  into  the  copper 
block,  care  being  taken  that  it  comes  exactly  flush  with  the  block,  no 
drops  or  beads  of  solder  appearing  around  the  joint.  To  do  this,  the 
'better  way  is  to  heat  the  copper  block  very  hot  and  thoroughly  tin  the 
inside  of  the  hole  and  fill  it  with  solder,  which,  if  the  block  is  hot  enough, 
will  remain  melted.  Having  previously  tinned  the  outside  of  the  wire, 
insert  it  in  the  hole  thus  prepared,  pushing  it  straight  in  until  the  proper 
distance  has  been  reached.  Be  very  careful  not  to  pull  it  outward  unless 
the  -solder  does  not  fill  well  around  the  hole  where  it  enters,  because  any 
outward  pull  of  the  wire  would  draw  a  meniscus  of  solder  around  it  which 
would  be  undesirable.  The  length  of  wire  between  the  two  connecting 
blocks,  as  shown  in  Fig.  222,  should  be  exactly  one  meter  long.  This 
should  be  most  carefully  adjusted  for  accuracy. 


CHEAP  TESTING  SET 


195 


-" 


M — 


FIG.   221.  —  COPPER  BAR. 


FIG.   223.—  METHOD  OF  FASTENING  TERMINAL  BLOCKS. 


FIG.   224.  —  WEIGHTED  INDEX. 


-<-<fcO 


FIG.   225.—  RESISTANCE  COIL. 


HOr 


|  1  1  Ml  1  I  ^ 


-^   ""JIM   I   |i   MI|IIM|IIU 
.°r  «•>  65"  ja 


FIG.   226.— IMPROVED  BRIDGE. 


ELECTRICAL   DESIGNS 

The  copper  blocks  are  provided  with  binding  screws  at  their  terminals 
for  the  convenient  insertion  of  known  and  unknown  resistances,  and  are 
secured  to  the  board  as  shown  in  Fig.  223  by  screws  passing  up  from  bc- 
iieath.  The  blocks  carrying  the  wires  are  arranged  to  hold  it  so  that  it 
will  be  free  from  all  kinks  and  twists,  but  not  to  put  any  mechanical  strain 
upon  it,  which  might  change  its  resistance ;  in  ether  words,  the  wire 
should  be  straight,  not  stretched.  A  little  platform  of  thin  wood  or  paste- 
board should  now  be  built  up  underneath  the  wire,  so  that  it  will  support 
it  throughout  its  length,  and  on  the  top  of  this  strip  of  pasteboard  should 
be  pasted  a  meter  scale ;  if  the  wire  has  been  accurately  adjusted,  it  will 
exactly  fit  between  the  terminal  blocks.  If  these  directions  and  the  draw- 
ings are  carefully  followed,  the  result  will  be  a  bridge  of  considerable 
range. 

To  use  this  instrument,  a  source  of  e.  m.  f.  is  connected  to  the  end 
blocks  and  a  galvanometer,  or,  as  shown,  a  telephone  receiver,  is  con- 
nected to  the  long  middle  block,  and  its  other  terminal  to  a  weighted 
index,  which  can  be  moved  along  the  board  between  the  wire  and  the 
long  copper  rod  and  make  contact  with  the  former  at  any  point  desired. 
This  index  is  preferably  made  cf  a  brass  rod,  one  end  of  which  has  been 
filed  up  to  a  V-shape  and  drawn  out  to  a  sharp  point,  as  shown  in  Fig. 
224.  The  other  "end  cf  the  brass  rod  is  conveniently  shaped  up  into  a 
binding  post,  to  which  the  detector  terminal  may  be  atttached.  The 
weight  may  be  a  piece  of  lead  cast  about  it  and  suitably  shaped.  The 
sharp  edge  of  the  brass  should  be  very  soft  in  order  not  to  mar  the  wire 
at  the  point  where  it  rests  upon  it,  and  for  that  purpose  should  be  an- 
nealed by  heating  it  in  a  flame  and  allowing  it  to  cool  slowly.  A  few 
standard  resistances  should  be  provided ;  one,  ten  and  one  hundred  ohms 
will  be  sufficient.  These  may  be  conveniently  made  by  measuring  off  the 
proper  amount  of  insulated  wire  with  a  testing  set,  coiling  it  about  a  small 
wooden  block  and  inserting  it  in  a  short  piece  of  large  brass  tubing,  whicli 
is  then  filled  with  parafnne.  These  coils  may  be  made  of  moderately 
fine  wire  and  stout,  projecting  terminals  of  as  nearly  no  resistance  as  may 
be,  led  out  through  the  paraffine  for  purposes  of  connection.  Fig.  225 
shows  a  section  of  such  a  coil  prepared  for  use. 

The  known  resistance  is  selected  as  nearly  as  possible  equal  to  the  tin- 
known  resistance  to  be  measured.  For  instance,  if  we  are  measuring  a 
series  of  field  coil  we  know  its  resistance  to  be  at  most  but  a  fraction  of  an 
ohm,  consequently  we  should  use  the  lowest  resistance  coil  that  we  have, 
which  in  this  case  would  be  one  ohm.  Similarly,  in  measuring  a  shunt 
field  coil  whose  resistance  may  be  16  or  20  ohms,  we  should  use  the  ten- 


CHEAP  TESTING  SET  i«;7 

ohm  coil,  and  so  on.  The  known  resistance  is  connected  across  the  bind- 
ing1 posts  at  one  end  of  the  bridge,  and  the  unknown  resistance  is  similarly 
connected  at  the  other.  A  source  of  e.  m.  f.,  two  or  three  dry  cells  will 
answer,  should  be  connected  from  A  to  B.  The  weight  index  is  adjusted 
along  the  wire  for  a  position  where  no  deflection  of  the  galvanometer  oc- 
curs, and  the  point  is  read  on  the  meter  scale. 

If  a  telephone  is  used  instead  of  a  galvanometer,  it  is  best  to  interpose 
a  key  in  the  circuit,  as  shown  in  Fig.  222.  The  procedure  is  to  then 
seek  a  point  on  the  scale  where  the  opening  and  closing  of  the  key  pro- 
duces no  click  in  the  receiver. 

Multiply  the  known  resistance  by  the  length  of  the  wire  on  the  un- 
known side  and  divide  it  by  the  length  of  wire  on  the  known  side,  and  the 
result  will  be  the  value  of  the  unknown  resistance. 

The  bridge  that  has  just  been  described  is  rather  a  clumsy  construction, 
being  nearly  4  ft.  long,  but  it  can  be  modified  as  follows,  and  the  result 
will  be  more  convenient : 

A  board  12  ft.  x  14  ins.  is  provided  with  blocks  of  copper,  as  shown  in 
Fig.  226.  Four  wires,  each  25  centimeters  long  (about  10  ins.),  should 
be  soldered  into  them,  as  shown  in  the  diagram,  the  blocks  at  the  end  be- 
ing shaped  as  shown  so  as  to  be  brought  up  conveniently  near  a  third 
one.  Assuming  the  blocks  to  be  of  negligible  resistance,  the  wire  must 
then  be  divided  into  four  lengths  of  25  scale  divisions  (one  meter)  each, 
and  thus  a  more  compact  and  satisfactory  instrument  will  be  obtained. 


CHAPTER  XXIV. 


CONSTRUCTION    AND    USE    OF    A    PHOTOMETER. 


In  the  photometer  here  described,  and  for  the  construction  and  use  of 
which  directions  are  given,  an  attempt  has  been  made  to  gather  into  one 
instrument  the  good  points  of  a  number,  and  at  the  same  time  to  avoid 
their  defects.  In  this  an  especial  indebtedness  is  owed  to  a  portable  pho- 
tometer made  by  the  Electric  Motor  and  Equipment  Company,  of  New- 
ark, N.  J.,  which  must  be  here  acknowledged. 

For  the  construction  shown  no  materials  are  required  which  are  not 
readily  procurable  at  small  cost,  nor  are  there  any  processes  of  manu- 


C*\ 


FIG.    227. — THE  PHOTOMETER  COMPLETE. 

facture  involved  which  are  beyond  the  capabilities  of  a  man  with  very 
simple  tools  and  ordinary  mechanical  ability.  In  many  things  some  sim- 
ple change  might  make  a  more  finished  instrument,  but  more  machine 
work  would  be  involved.  Such  modifications  will  readily  suggest  them- 
selves to  anyone  undertaking  the  construction. 

A  general  drawing  of  the  instrument  is  given  in  F,ig.  227.  The  frame, 
which  supports  everything,  is  of  J^-'m.  white  pine  3  ins.  wide,  set  together 
in  an  open  rectangle  5  ft.  iol/2  in.  long  by  6^4  ins.  wide  over  all.  This 
frame  may  be  supported  at  a  convenient  height  by  brackets  on  any  side 
wall.  At  the  left  is  an  argand  gas  burner,  in  front  of  which  is  a  board 
with  an  adjustable  slot  cut  in  it,  the  slot  being  directly  in  front  of  the  ar- 
flame.  At  the  other  end,  on  a  suitable  stand,  is  the  incandescent 


CONSTRUCTION  AND   USE  OF  A  PHOTOMETER 


199 


lamp  whose  candle-power  is  to  be  measured.  Between  them  is  a  frame 
on  which  travels  a  car  containing  a  piece  of  paper  set  in  a  plane  at  right 
angles  to  a  line  joining  the  two  lights.  This  paper  has  a  grease  spot  on 
it,  and  in  use  the  car  is  moved  backward  and  forward  until  a  place  is 
found  where  the  grease  spot  disappears.  The  candle-power  of  the  lamp 
is  then  read  off  on  a  scale  immediately  below  the  car.  For  adjusting 
the  e.  m.  f.  on  the  lamp  terminals  a  rheostat  is  provided,  shown  between 
the  incandescent  lamp  and  the  track  for  the  car  at  X  in  the  drawing. 
The  instrument  is  in  principle  a  Bunsen  photometer. 

In  Fig.  228  the  car  is  shown  in  detail.  The  car  proper  is  of  wood, 
open  in  front,  closed  at  the  back,  with  ends  cut  away  in  openings  2l/2  ins. 
in  diameter.  Half  way  between  these  two  ends  is  the  screen — seen  in 


x 

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FIG.    228. — THE   CAR   IN   DETAIL. 

edge  view  at  A.  This  is  of  two  pieces  of  sheet  metal  (see  D),  about  No. 
14  or  No.  15  B.  &  S.  gauge,  in  both  of  which  there  is  cut  a  circular  hole 
2  ins.  in  diameter.  Between  these  pieces  is  put  the  paper  with  grease 
spot,  as  shown,  and  the  two  plates  held  together  with  small  screws  so  the 
paper  and  grease  spot  can  readily  be  renewed  when  necessary.  The 
compound  plate,  D,  slides  in  place  from  the  front  of  the  car  in  small 
wooden  guides  as  at  A. 

To  the  right  of  the  frame  D,  in  the  figure,  is  a  section  of  the  car  taken 
half  way  between  the  top  and  bottom.  The  two  blocks,  F,  F,  are  wooden 
ones,  extending  from  bottom  to  top  of  the  car  on  which  pieces  of  mirror 
are  mounted.  These  mirrors  should  be  of  good  quality,  as  near  alike  as 
possible,  and  covering  a  large  part  of  the  surfaces  (7,  so  that  an  observer 


200  ELECTRICAL  DESIGNS 

sees  in  each  of  them  the  reflection  of  the  paper  and  grease  spot — one  side 
5i  the  paper  in  one  and  the  other  side  in  the  other.  The  exact  angle 
at  which  these  mirrors  should  be  placed  will  best  be  determined  in  each 
case  by  trial.  As  shown,  the  results  should  be  perfectly  satisfactory.  The 
piece  B,  between  car  proper  and  base,  on  which  the  wheels  are  mounted, 
is  to  be  of  lead.  It  may  be  cored  out  some,  but  is  to  give  weight  and 
stability  to  the  car  on  the  track  and  so  must  be  heavy.  The  base  to 
which  wheels  C  are  secured  is  of  wood  j£-in.  thick  and  shaped  to  carry 
the  lead  block  and  three  wheels  as  shown,  two  running  on  the  forward 
track  and  one  on  the  rear  track.  These  wheels  are  simply  porcelain  in- 
sulators or  knobs,  which  any  central  station  will  be  likely  to  have  in 
stock,  or  which  may  readily  be  procured  from  any  dealer  in  electrical  sup- 
plies. They  run  on  wood  screws  as  axles.  If  they  are  turned  from  iron 
or  brass,  B  may  be  made  of  wood.  The  three  parts  making  up  the  car 
are  put  together  with  two  slender  bolts  passing  down  through  all,  from 
the  car.  E,  in  another  part  of  the  figure,  shows  a  brass  plate  of  about  No. 
12  or  14  gauge,  screwed  to  the  top  of  the  car  and  with  light  stiff  wires 
(about  No.  10)  soldered  to  it  and  projecting  forward.  To  each  of  these 
wires  is  hung  a  curtain  of  black  drilling,  43/2  or  5  ins.  wide  and  10  or  12 
ins.  long,  hung  as  shown  at  W  (Fig.  227).  These  curtains  are  to  screen 
the  eyes  of  the  observer  from  the  lights  at  the  end  of  the  bar.  When  this 
car  js  finished,  it  must  be  painted  black  all  over.  Not  a  shiny  black,  but 
a  dull,  dead  black,  such  as  may  be  had  at  least  cost  probably  by  cutting  a 
little  lamp  black  with  turpentine  and  adding  just  enough  shellac  varnish 
to  make  the  black  stick,  but  not  get  glossy. 

Before  taking  up  another  part  of  the  photometer,  a  few  words  as  to 
methods  of  making  the  screen  will  be  well  put  in.  The  sort  of  paper 
used  is  not  of  great  importance.  It  must  be  white  and  quite  nearly 
opaque.  Preferably  both  sides  should  be  as  nearly  alike  as  possible.  The 
grease  spot  must  be  as  transparent  as  possible,  have  a  very  clearly  defined 
edge  and  the  more  edge  the  better.  In  Fig.  231  there  is  shown  a  star 
which  is  a  good  size  to  use  and  which  has  plenty  of  edge.  To  make  the 
grease  spot,  cut  the  star  out  of  brass  plate  (say  j/s-in.  thick)  and  mount  it 
on  a  rod  set  perpendicular  to  the  plane  of  the  star.  Melt  some  paraffine 
over  a  water  bath,  put  the  star  into  the  paraffine  and  let  it  warm  some ; 
then  take  it  out,  let  it  drain  and  set  it  down  on  the  paper  selected.  A 
number  of  grease  spots  having  been  made,  pick  out  the  best  for  use.  I 
have  found  a  typewriter's  paper,  imitation  linen,  smooth,  and  rather  thin, 
quite  satisfactory — care  being  taken  not  to  use  any  part  containing  a 
water  mark. 


CONSTRUCTION  AND   USE  OF  A  PHOTOMETER 


201 


The  car  runs  on  a  track  of  which  a  satisfactory  idea  may  be  had  from 
Figs.  227  and  232.  The  two  tracks  are  of  common  J/2-in.  iron  pipe, 
shown  at  P  (Fig.  232),  slipped  through  end  pieces,  whose  form  and  size 
are  shown  in  the  same  figure ;  and  finished  by  iron  caps  screwed  on  each 
end.  It  will  be  necessary  to  pick  out  good  smooth  pieces  of  pipe  for  the 
purpose  and  perhaps  do  a  little  filing,  so  that  in  use  one  will  not  uncon- 
sciously tell  by  the  "feel  of  the  track"  when  the  car  is  in  some  certain 
position  along  it.  Q,  in  Fig.  232,  is  a  strip  of  7-i6-in.  wood  which  is  in 
front  of  the  pipe  tracks  in  Fig.  227,  and  is  to  have  the  scale  for  reading 
candle-power  mounted  on  it. 

The  framework  must  be  mounted  rigidly  in  place  and  the  distances  be- 
tween it  and  the  end  fixtures,  as  well  as  the  distances  between  them  (the 


-"4| 

yxy. 

rfr 

r  , 

J\f»                                                 T 

r 

1  1 
1  1 

V 

I 

? 

i*-  ^ 
—  ?/?*' 

\  --  '*"  —  1 


Iff 


FIGS.    229   AND   230. — DETAILS    OF   RHEOSTAT  AND      FIG.    231. — STAR   OF   SIZE 
CONTACT  MAKER.  FOR   GREASE   SPOT. 

screen  and  the  incandescent  lamp)  made  exactly  as  shown.  It  is  on  the 
accuracy  of  these  measurements  that  the  value  of  the  table  given  on  page 
207,  determining  the  candle-power  scale,  depends.  The  scale  of  candle- 
power,  to  be  mounted  on  the  strip  in  front  of  the  iron  tracks  as  before 
mentioned,  may  be  put  on  a  piece  of  cardboard  tacked  to  the  board,  the 
divisions  laid  out  as  per  table  on  page  207  with  waterproof  India  ink  and 
then  the  whole  shellacked  over  to  preserve  it  and  prevent  weather 
changes  from  warping  the  cardboard  out  of  shape. 

The  table  is  to  read  candle-power  from  eight  to  thirty.  This  is  an 
abundant  range  for  i6-c.p.  lamps.  Half  candle-powers  may  also  be 
marked  off  by  divided  distances  between  marks  on  the  scale,  except  for 
values  when  these  are  given  in  the  table.  A  pointer  is  attached  to  the  car 
directly  under  the  greased  paper  (as  per  Fig.  227)  by  which  to  read  car 
position  and  so  candle-power. 


202 


ELECTRICAL  DESIGNS 


At  the  left  end  of  the  frame  is  an  argand  gas  burner  (See  Fig.  227). 
An  oil  lamp  might  replace  the  argand  burner,  but  if  gas  is  available  it  will 
be  much  more  convenient  to  use.  In  the  figure  the  argand  burner  is 
shown  quite  near  the  slot  board,  so  near  indeed  that  it  is  probable  the 
f  board  will  have  to  be  covered  with  asbestos  paper  on  the  side  next  it. 
To  procure  the  results  mentioned  in  Part  II.,  it  will  be  necessary  to  keep 
the  burner  up  clos?e.  The  exact  position  of  the  gas  burner  is  not  of  im- 
portance except  that  it  must  be  directly  behind  the  slot  when  viewed  from 
the*  car.  The  screen  is  shown  in  detail  in  Fig.  233.  It  is  arranged  with 
a  slider  to  control  the  width  of  the  slot  S,  which  is  i  in.  x  2  ins.  at  largest 
opening.  If  facilities  are  at  hand  it  will  be  well  to  provide  a  screw  and 

/ v 


FIG.    22. 


4 


.  —  DETAIL   OF   TRACK   AND    SCALE 
ARRANGEMENT. 


FIG.    233. DETAILS    OF   SCREEN. 


nut  on  the  slider  to  control  the  width  of  the  slot  within  small  limits  and 
readily.     Bevel  off  the  slot  in  fashion  shown  at  R. 

At  the  right  hand  end  of  the  frame  is  the  standard  to  hold  the  incan- 
descent lamp,  connection  board  and  rheostat.  An  ordinary  lamp  socket 
has  a  J^-in.  pipe  screwed  into  it  and  this  drops  into  a  ^-in.  pipe  in  a  cast 
iron  base  through  a  ^j-in.  cap  drilled  at  the  end  and  with  set-screw  put 
through  to  hold  the  lamp  rod  at  any  height.  The  lamp  cord  is  brought 
from  the  socket  as  shown  and  its  ends  carried  to  the  two  binding  posts, 
Pa  and  Pa,  respectively.  The  mains  are  connected  to  the  binding  posts 
at  M  (Fig.  227),  voltmeter  at  posts  between  which  the  letter  V  is  put,  and 
ammeter,  if  used,  at  posts  on  each  side  of  Am,  as  marked.  Otherwise 
this  ammeter  gap  is  bridged  by  a  copper  link.  Two  connecting  straps 
are  shown  on  the  face  of  the  frame  and  other  connections  are  completed 
inside,  so  that  the  circuit  is  from  one  post  at  M  to  PS,  then  through  the 


CONSTRUCTION  AND  USE  OF  A  PHOTOMETER  203 

lamp  to  PS,  to  brass  rod  marked  H  (Fig.  229),  through  the  contact  into  the 
rheostat  coil,  then  to  P,  and  so  out  at  the  other  side  of  connection  for  the 
mains.  The  voltmeter  connected  thus  reads  e.  m.  f.  on  the  lamp  termi- 
nals and  the  ammeter,  both  the  current  through  the  lamp  and  voltmeter, 
so  that  if  this  latter  takes  appreciable  current,  as  it  usually  does,  allow- 
ance must  be  made,  unless  the  voltmeter  is  always  off  circuit  when  the 
ammeter  reading  is  taken. 

For  the  rheostat  construction  see  Figs.  229  and  230.  7  is  a  round 
wooden  core  which,  with  end  pieces,  is  preferably  made  of  hard 
wood.  Over  7  is  a  thin  covering  of  asbestos  paper,  put  on 
smooth  and  tight,  and  shellacked  to  place.  Over  14^4  inches  of 
length  at  the  middle  of  the  rod  is  wound  a  tight  spiral  of  No.  20 
German  silver  double  cotton  covered  wire  also  shellacked  to  place.  This 
coil  may  be  secured  at  its  ends  by  tieing  it  down  as  with  an  armature  coil 
or  by  soldering  together  the  last  two  or  three  turns  at  each  end.  Along 
one  side  of  the  cylinder  so  formed,  after  the  shellac  has  dried,  and  under 
the  brass  rod  H  (J4  in.  thick  x  y2  in.  wide),  the  insulation  on  the  outside 
is  to  be  rubbed  off  with  sandpaper,  making  a  path  over  the  bared  wire,  $/& 
or  J4  ms-  wide.  This  leaves  each  turn  insulated  from  its  neighbors,  but 
makes  it  possible  to  connect  readily  with  any  turn  of  the  solenoid.  Bridg- 
ing the  distance  from  H  to  the  wire  on  7,  and  making  the  contact  just 
mentioned,  is  the  contact  device  of  Fig.  230.  This  is  a  brass  body  K,  cut 
from  J^-in.  square  rod,  with  an  opening  M,  which  rides  easily  on  the 
rod  77.  Secured  to  it  are  German  silver  springs,  as  shown  within,  to 
make  contact  with  the  rod,  and  outside  to  make  contact  with  the  turns 
of  wire  on  the  coil.  A  bolt  like  that  at  L  can  be  had  from  almost  any  old 
lamp  socket.  It  had  better  be  very  securely  put  in  place.  As  the  success 
of  the  rheostat  depends  on  this  contact  device,  it  must  be  carefully  made 
and  the  lower  spring  piece  divided  on  each  side  as  at  Ar,  so  there  may 
be  four  contacts  on  the  rheostat  wires.  With  this  rheostat  the  volts  on  a 
i6-candle-power  no-volt  lamp  can  be  controlled  over  a  range  of  ten 
volts  by  almost  imperceptible  steps  and  one  ampere  can  be  put  through 
it  continuously  without  dangerous  heating.  The  whole  thing  is  held  to- 
gether by  wood  screws  from  the  ends,  0,  into  the  rod,  7,  and  then  screwed 
in  place  at  the  side  of  the  frame. 

After  the  photometer  is  all  put  together  and  mounted  on  a  side  wall, 
the  whole  thing  must  be  given  a  couple  of  coats  of  a  dead-black  paint,  and 
the  wall  immediately  behind  the  instrument  painted  the  same  way.  A 
dark  room.need  not  be  provided.  All  very  bright  lights,  however,  must 
be  absent  from  the  vicinity  of  the  instrument  or  turned  out  when  it  is  in. 
use. 


204  ELECTRICAL  DESIGNS 

HOW  TO  USE  THE  PHOTOMETER. 

Two  needs  must  be  supplied  before  any  work  can  be  done  with  the 
finished  instrument.  There  must  be  standard  incandescent  lamps  and  a 
thoroughly  reliable  voltmeter.  The  incandescent  lamps  may  be  pur- 
chased from  any  of  the  larger  lamp  companies,  marked  with  the  voltage 
at  which  they  give  16  candle-power,  and  from  what  direction  they  must 
be  viewed  to  give  this  candle-power.  Sixteen-candle-power  standards 
must  be  used.  By  a  thoroughly  reliable  voltmeter  is  meant  one  which 
has  no  volts  at  its  terminals  when  the  needle  points  to  this,,  that  always 
conies  to  a  no-volt  reading  when  no  volts  is  applied,  and  which  can  be 
read  to  one-fifth  of  a  volt.  Unless  such  a  voltmeter  is  used,  only  very 
rough  candle-power  measurements  can  be  made.  Errors  as  much  as 
]/2  candle-power  to  I  candle-power  in  16  will  come  in,  due  to  this  cause 
alone,  with  a  voltmeter  in  use  whose  readings  are  slightly  in  doubt.  The 
circuit  used  must  be  one  on  which  the  e.  m.  f.  at  the  lamp  can  be  held 
steady  for  a  similar  reason.  An  ordinary  i6-c.p.  lamp  changes  candle- 
power  about  one  unit  per  volt  change  in  applied  e.  m.  f.  at  the  normal  or 
rated  e.  m.  f. 

Having  the  standards  and  the  voltmeter  for  the  work,  proceed  to 
measurements  this  way.  Two  men  are, required,  one  to  watch  the  volt- 
meter and  change  lamps,  the  other  to  work  the  car.  Connect  the  circuit, 
put  one  of  the  standards  in  the  socket,  set  it  at  its  marked  voltage  and 
turned  so  one  views  it  from  the  photometer  car  in  a  direction  which  makes 
its  candle-power  16,  light  the  argand  burner,  put  the  car  so  its  index  is  at 
mark  16,  and  adjust  the  slot  at  the  argand  until  the  grease  spot  disap- 
pears. The  argand  lamp  has  now  become  the  standard  and  is  assumed 
to  stay  at  fixed  candle-power  for  further  work.  Next  remove  the  stand- 
ard lamp,  put  in  one  whose  candle-power  is  to  be  determined,  bring  it 
to  a  proper  applied  e.  m.  f.,  and  while  one  man  reads  the  voltmeter  and 
keeps  the  lamp  at  a  constant  terminal  potential  difference,  the  other  one 
moves  the  car  backward  and  forward,  seeking  a  place  where  the  grease 
spot  cannot  be  distinguished  from  the  surrounding  paper,  and  when  the 
place  is  found  he  reads  the  candle-power  of  the  lamp  now  in  the  socket. 
So  work  is  proceeded  with,  a  return  being  made  periodically  to  the  stand- 
ard to  check  the  argand  burner,  which  is  the  working  standard,  and  per- 
haps also  to  other  lamps  as  well  to  procure  check  readings  of  their  candle- 
power.  A  check  on  the  argand  by  the  incandescent  lamp  standard  is  of 
course  made  by  putting  the  car  at  16  candle-power,  adjusting  the  incan- 
descent lamp  in  position  and  applied  e.  m.  f.,  and  observing  whether  the 
grease  spot  is  still  invisible.  The  one  handling  the  car  must  keep  his 


CONSTRUCTION  AND   USE  OF  A  PHOTOMETER 


205 


•*.c 


eye  solely  for  observations  of  the  grease  spot  and  will  not  be  able  to  do 
good  work  until  he  has  gotten  accustomed  to  it  by  ten  minutes'  prelim- 
inary work.  Also  a  person  without  experience  in  photometric  measure- 
ments will  not  be  able  to  get  good,  that  is,  concordant  and  correct  results. 
A  practiced  observer  will  have  results  agree  with  this  photometer  so  that 
individual  observations  never  differ  by  more  than  a  half-candle-power 
in  sixteen. 

It  will  be  very  unusual  for  the  grease  spot  to  simultaneously  disap- 
pear from  both  of  the  images  in  the  mirrors  for  any  one  position  of  the 
car.  This  is  mainly  due  to  difference  in  the  color  of  the  lights  used.  Old 
incandescent  lamps  are  less  troublesome  than  new  ones  in  this  respect.  In 
making  a  reading  one  should  set  the  car  to  a  position  where  the  differ- 
ence in  tint  of  spot  and  field  is 
the  same,  whichever  mirror  he 
views. 

It  must  be  observed  that 
when  making  measurements  no 
reversing  of  car  is  to  be  done, 
nor  is  there  necessarily  a  dark 
room.  Simply  avoid  lights  or 
white  surfaces  before  the  eyes 
of  the  observer  and  have  the 
lights  in  the  vicinity  of  the  in- 
strument as  few  as  possible,  of 
as  small  candle-power  as  possi- 
ble, and  constant  in  position  and 
candle-power.  If  one  can  just 
manage  to  read  newspaper  print 
from  the  surrounding  lights 
it  will  be  dark  enough.  Avoid  light  most  carefully  at  the  incandescent 
lamp  end. 

If  no-volt  lamps  are  to  have  their  candle-power  determined  it  com- 
monly happens  that  the  dynamos  run  at  no  volts.  Hence  it  is  well  nigh 
impossible  to  get  no  volts  at  the  photometer  and  in  any  event  the  rheo- 
stat would  be  useless.  One  should  have  a  few  small  storage  cells  to  put  in 
series  in  the  circuit  on  this  account,  to  raise  the  e.  m.  f.  to  about  120,  and 
then  there  is  abundant  opportunity  to  procure  the  no  volts  steadily,  even 
tinder  considerable  fluctuations  in  the  e.  m.  f.  on  the  circuit. 

A  i6-c.p.  lamp  does  not  measure  16  candle-power  when  viewed  from 
any  direction.  There  is  a  good  deal  of  variation  in  candle-power 
according  to  the  direction  from  which  one  views  the  lamp,  and 


FIG.    234. — DIAGRAM   OF   HORIZONTAL    CANDLE 

POWERS    ON    TWO    LAMPS,    ONE    WITH 

COILED,   THE   OTHER  WITH   PLAIN 

HORSESHOE  FILAMENT. 


206 


ELECTRICAL   DESIGNS 


especially  is  this  true  for  coiled  or  looped  filaments.  Even  the  candle- 
power  in  a  plane  at  right  angles  to  the  lamp  axis,  the  horizontal  candle- 
power  is  irregular.  One  can  readily  see  this  for  himself  by  holding  a 
piece  of  white  paper  near  a  lighted  lamp  and  observing  the  bright 
streaks  of  light  in  certain  directions.  The  kind  of  variation  which  exists 
is  shown  by  two  curves  of  horizontal  candle-power  plotted  together  in 
Fig.  234.  These  were  taken  from  two  lamps,  one  of  which  had  a  coiled 
filament  and  the  other  a  plain  U-form  filament. 

Most  lamp  manufacturers  are  now  rating  their  product  by  the  candle- 
power  measured  when  the  lamps 
are  rotating  on  a  vertical  axis  at 
1 80  r.  p.  m. — a  value  decided  on 
at  the  National  Electric  Light 
Convention  a  year  or  two  since. 
A  rotating  socket  is  not  included 
in  the  photometer  design  here 
given.  Fig.  235  shows,  how- 
ever, a  rotating  socket  recently 
built  for  use  in  the  Electrical 
Laboratory  at  Drexel  Institute 
here  shown,  which  might  re- 
place the  incandescent  lamp 
stand  in  the  design  given  in  this 
article.  The  one  shown  in  the 
cut  has  embodied  in  it  certain 
features  worth  noting.  There 
are  four  brushes  rubbing  on  four 
contact  rings.  Two  carry  cur- 
rent into  and  out  of  the  lamp 
through  a  socket  so  arranged 

that  the  voltmeter  connected  through  the  two  other  brushes  is  actually 
connected  to  the  lamp  terminals,  and  so  the  true  e.  m.  f.  on  the  lamps 
is  known.  To  maintain  a  steady  rate  of  rotation  the  little  motor  shown  is 
run  by  two  storage  cells,  given  up  to  this  duty  alone.  The  speed  of  the 
rotating  socket  is  also  under  control  and  can  be  varied  through  all  neces- 
sary limits  by  shifting  the  little  rubber-covered  pulley  to  different  radii 
under  the  large  rotating  disc,  which  is  mounted  on  the  stem  of  the  lamp 
whose  candle-power  (that  is,  mean  horizontal  candle-power)  is  to  be  de- 
termined. With  proper  tools  and  facilities  such  a  rotating  device,  or  one 
equivalent,  can  be  put  on  the.  photometer  here  described  and  so  a  very 
complete  instrument  be  had. 


FIG.    235. — A  VIEW  OF  A  ROTATING  SOCKET. 


CONSTRUCTION  AND   USE  OF  A  PHOTOMETER 


207 


The  standard  lamp  is  better  if  not  rotated.  Care  must  be  taken  that 
the  standard  is  not  viewed  from  a  position  near  to  one  like  C,  in  the  figure 
of  horizontal  distribution,  however,  where  the  turning  of  the  lamp 
through  only  five  degrees  varied  the  candle-power  by  more  than  five 
units.  A  plain  U-shaped  filament  is  much  the  best  for  a  standard  lamp. 


TABLE  FOR  CONSTRUCTION  OF  CANDLE- 

TABLE  FOR  CONSTRUCTION  OF  No.  2 

POWER  SCALE. 

CANDLE-POWER  SCALE. 

DISTANCE  FROM  SLOT 

Candle- 

DISTANCE  FROM  SLOT 

Candle- 

BOARD  IN  INCHES. 

power  to  be 

BOARD  IN  INCHES. 

power  to  be 
•»*•„  —  i-.~,-i  ,^, 

Exact  Value. 

Nearest 
64th. 

Marked  on 
Scale. 

Exact  Value. 

Nearest 
64th. 

Marked  on 
Scale. 

21.961 

2iU 

30 

24.027 

24* 

45 

22.198 

'     22^f 

29 

24.212 

24* 

44 

22.444 

22TV 

28 

24.400 

24sf 

43 

22.7 

22|f 

27 

24.593 

24if 

42 

22.967 

22f£ 

26 

24.794 

24U 

41 

23.246 

23^ 

25 

24.997 

25 

40 

23-537 

23tf 

24 

25.210 

25$ 

39 

23.842 

23B 

23 

25.429 

25|| 

38 

24.162 

24s5* 

22 

25.651 

25tt 

37 

24.498 

24^ 

21 

25.884 

4H 

36 

24.8525 

24if£ 

20 

26.124 

26^ 

35 

25.227 

25if 

19 

26.370 

26]/& 

34 

25.623 

25* 

18 

26.625 

26% 

33 

26.0435 

26* 

17 

26.890 

26H 

32 

26.491 

26H 

16 

27.163 

27A 

31 

26.97 

26j| 

15 

27.451 

27f| 

30 

27.4825 

27« 

14 

27.746 

273/ 

29 

28.035 

28A 

13 

28.052 

28¥8r 

28 

28.634 

28H 

12 

28.374 

28^8 

27 

29.285 

29ft 

II 

28.707 

28|| 

26 

29.634 

29>i 

10.5 

29.055 

29/T 

25 

30.000 

30 

10 

30.384 

3off       ' 

9-5 

30.790 

soli 

9 

31-219 

3i  ft 

8.5 

31-672 

341 

8     r" 

The  candle-power  of  8-c.p.  lamps  can  readily  be  obtained  with  this 
photometer  also.  Put  a  standard  8-c.p.  lamp,  at  some  marked  voltage, 
in  the  lamp  socket ;  put  the  car  at  16  candle-power  and  adjust  the  slot  at 
the  argand  burner  until  the  slot  disappears.  Then  proceed  to  measure 
the  8-c.p.  lamps  as  though  they  were  i6's,  halving  the  value  of  candle- 


2o8  ELECTRICAL   DESIGNS 

power  on  the  scale  in  each  case  for  their  real  candle-power.  A  similar 
method  may  be  used  also  in  determining  the  value  of  the  candle-power 
procured  from  32-c.p.  lamps.  Use  a  standard  32-c.p.  lamp  as  in  case  of 
the  standard  8-c.p.  above,  and  when  readings  are  made  on  the  candle- 
power  scale  double  the  value  obtained  in  each  case.  Unless  the  gas  used 
is  intrinsically  of  very  good  candle-power  and  the  argand  burner  is  put 
very  close  to  the  slot  board,  this  last  cannot  be  done  since  the  gas  burner 
will  not  give  enough  light.  If  it  is  a  possible  plan  carefully  avoid  any 
flickering  edges  of  flame  showing  through  the  slot  when  it  is  viewed  from 
the  car  position.  The  following  alternative  method  of  measuring  32-c.p. 
lamps  will  always  be  satisfactory. 

Set  up  a  second  lamp  socket  (number  2)  fifteen  (15)  inches  to  the 
right  of  the  one  already  provided  in  the  regular  construction,  or  seventy- 
five  (75)  inches  from  the  slot.  Construct  a  second  scale  (number  2)  under 
the  one  already  made  (number  i),  to  be  used  only  when  the  lamp  under 
test  is  in  the  more  distant  socket. 

This  scale  will  be  constructed  by  measurements  given  in  the  table 
on  page  207,  all  measurements  being  from  the  slot  before  the  argand 
lamp. 

Use  i6-c.p.  standard  in  socket  number  I,  put  the  car  at  16  candle- 
power  on  scale  number  I,  and  set  the  argand  lamp  by  it  so  the  spot, 
disappears.  Then  put  the  32-c.p.  lamp,  whose  candle-power  is  desired 
in  number  2  socket,  and  measure  its  candle-power  in  the  usual  way, 
but  reading  its  candle-power  on  number  2  scale.  If  the  two  sockets  are 
connected  in  parallel  they  will  be  ready  for  operation  alternately  at  any 
time,  the  same  instruments  and  rheostat  being  used  without  any  change 
in  connections. 


CHAPTER  XXV. 


CONSTRUCTION  OF  A  SIMPI^  STORAGE  BATTERY. 


The  accompanying  engravings  show  the  construction  of  a  plate  and 
a  single  cell  of  storage  battery  of  the  Faure  type,  which  may  be  built  with 
no  tools  beyond  a  pair  of  heavy  tinners'  shears,  a  small  punch  and  a  slit- 
ting saw  of  the  sort  used  for  cutting  thin  metals. 

Each  plate  is  made  up  of  twelve  strips  of  lead  cut  to  the  shape  shown 
in  Fig.  236.  This  strip  is  0.075  m-  thick,  8  ins.  long,  over  all,  and  ]4>  in. 
wide.  Midway  on  each  edge  is  cut  a  square  slot,  s,  3-16  in.  wide  and 
deep ;  the  ends  of  the  strip  are  cut  down  to  3/2  in.  in  width  for  a  distance 
of  %  in.  from  each  extreme  end,  and  two  l/%  in.  holes  are  punched  in  the 
narrow  part  of  the  strip,  as  shown.  The  edges  are  folded  along  the  dot- 
ted lines  until  the  end  view  of  the  strip  looks  like  B,  Fig.  236. 

Next  three  rubber  forks,  like  D,  Fig.  237,  are  provided  for  each  plate 
(not  each  strip,  but  each  group  of  twelve  strips).  Each  fork  is  made 


A  B. 

FIG.    2^6. — SHAPE  OF  STRIP. 

from  a  strip  of  hard  rubber,  3-16  in.  thick,  ^  in.  wide,  and  75/2  ins.  long; 
the  slit  down  the  center  must  be  just  wide  enough  to  admit  the  thickness 
of  the  lead  strips  forming  the  plate  (A,  Fig.  236)  with  no  "lost  motion," 
and  must  stop  exactly  i  in.  from  the  lower  end  of  the  rubber.  When 
these  rubber  forks  are  ready,  a  plate  is  built  up  in  three  of  them  as  fol- 
lows :  Cut  one  of  the  lead  strips  in  two  longitudinally,  exactly  down  its 
center,  and  assemble  the  strips  on  edge  in  the  rubber  fingers  so  that  the 
end  view  of  the  lead  part  of  the  structure  looks  like  E,  Fig.  237.  Fig. 
238  shows  the  face  view  of  the  complete  plate.  The  halves  of  the  strip 
that  was  cut  longitudinally  go  at  the  top  and  bottom  of  the  plate,  and  are 
indicated  by  a  and  b,  Figs.  237  and  238. 


210 


ELECTRICAL   DESIGNS 


After  the  strips  are  assembled  in  the  retaining  forks,  D,  D,  D,  tie  the 
upper  ends  of  the  ringers  tightly  with  lead  wire  so  that  they  clamp  the 
plate ;  this  tie-wire  should  go  just  above  the  top  strip,  at  s,  and  it  is  im- 
perative that  no  material  other  than  lead  be  used,  unless  very  strong, 
short  rubber  bands  are  obtainable,  in  which  case  they  may  be  used.  Then 
cut  two  strips  of  lead,  C,  c,  each  y§  in.  wide,  and  of  the  same  thickness  as 
the  lead  strips  composing  the  plate ;  the  longer  one,  C,  is  8^  ins.  long, 
and  the  other  one,  c,  6j^  ins.  long.  Rivet  one  of  these  to  all  the  ends  of 
the  plate  strips  at  one  extremity  of  the  plate  and  the  other  one  to  the  ends 
at  the  opposite  extremity,  using  lead  rivets.  The  engraving  shows  holes 
punched  in  the  connecting  strips,  C,  c,  to  correspond  with  those  in  thfe 


> 

— 

^* 

0       .1 

e      o- 

0    c 

0      0 

7~] 

0       c 

o     o 

»  •! 

Tg 

•  •! 

0        0 

o    e 

TT] 

0       O 

0      0 

13 

•s—  TI 

FIG.  237. — RUBBER   FORK 
AND   PLATE   EDGE. 


FIG.    238. — COMPLETED 


ends  of  the  plate  strips,  A.  Bend  the  upper  ends  of  the  long  connectors, 
C,  about  half  an  inch  from  the  end,  at  right  angles  to  the  main  body  of  the 
strip ;,  the  bending  point  is  indicated  by.  xy  Fig.  238.  The  edges  of  the 
plate  strips  which  were  bent  at  an  acute  angle  to  the  body  strips  (B,  Fig. 
236)  are  designed  to  serve  as  shelves  to  hold  the  paste  or  active  material 
which  is  to  be  applied  to  each  plate.  The  process  of  pasting  will  be  de- 
scribed later  on. 

After  the  paste  has  been  put  on  the  plates  and  has  hardened,  nine 
plates  (four  positive  and  five  negative)  are  assembled  in  a  glass  jar,  as 


CONSTRUCTION  OF  A  SIMPLE  STORAGE  BATTERY. 


211 


shown  by  the  plan  view,  Fig.  239,  where  A  is  the  lead  portion  of  the  out- 
side plates;  P,  the  paste,  or  active  material;  D,  the  rubber  retaining  fin- 
gers ;  /,  the  glass  jar,  and  C,  c,  the  vertical  connecting  strips.  The  strips, 
C,  are  shown  coming  straight  upward  instead  of  bent  over,  to  avoid  ob- 
scuring the  view  of  the  plate  ends.  The  rubber  fingers,  D,  in  addition 
to  holding  the  twelve  strips  composing  each  plate  in  their  proper  posi- 
tions, serve  to  prevent  the  plates  themselves  from  buckling  and  shifting  in 
the  jar.  The  edges  of  these  rubber  ringers  are  shown  slightly  separated, 
but  as  a  matter  of  fact,  they  must  abut  each  other  so  as  to  keep  the  plates 
firmly  in  position.  The  jar,  J,  is  rectangular,  and  should  be  8*4  ins.  long, 
5)4  ins.  wide,  and  ^/2  ins.  deep,  inside  measurement.  None  of  its  di- 
mensions can  be  smaller  than  specified ;  if  the  width  or  length  be  greater, 
a  strip  of  wood  may  be  used  to  bring  it  down  to  the  figure  desired.  A 


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FIG.   239. — PLAN  VIEW   OF   BATTERY   CELL. 

sheet  of  soft  rubber  must  be  laid  against  each  end  and  side  wall  of  the  jar, 
and  the  plates  must  fit  snugly  against  the  rubber  sheets  so  that  they  will 
not  shift  in  the  jar ;  soft  rubber  must  be  used  for  the  wall  sheets  in  order 
that  the  plates  may  expand  without  tending  to  buckle. 

As  above  stated,  each  cell  contains  four  positive  plates  and  five  nega- 
tives; the  two  outside  plates  are  negative,  and  have  paste  on  only  one  side 
each,  as  the  sides  next  to  the  wall  of  the  jar  are  not  active.  The  terminal 
strips,  C — ,  on  the  right  hand  end  of  the  cell  are  all  negative;  those 
marked  C  +  on  the  left  hand  end  are  the  positives,  as  their  signs  indicate. 


2-2  ELECTRICAL   DESIGNS 

The  short  strips,  c,  serve  simply  to  connect  the  parts  of  the  plates  to  each 
other,  and  should  not  project  above  the  upper  edges  of  the  plates. 

All  the  positive  ends,  C  +,  at  the  left  are  riveted  to  a  lead  strip,  T, 
which  is  laid  along  the  top  of  the  bent-over  ends  of  the  strips,  C  +.  The 
negative  connectors  are  similarly  riveted  to  the  other  strip,  T  2,  and  these 
two  strips  form  the  terminals  of  the  complete  cell.  The  arrows  at  the 
corners  of  the  jar  indicate  the  direction  in  which  the  ends  of  the  terminals 
are  led  from  the  cells  to  connect  to  an  adjoining  cell  or  to  leading  wires, 
as  the  case  may  be. 

Each  cell  of  battery  of  the  above  dimensions,  when  properly  pasted 
and  "formed/'  will  give  an  electro-motive  force  of  about  2  volts  during 
the  greater  part  of  its  discharge,  and  it  may  be  discharged  at  the  rate  of 
18  to  20  amperes.  To  operate  any  of  the  small  motors  described  by  the 
writer  on  pages  i  to  14,  inclusive,  four  of  these  cells  will  be  required 
(The  battery  winding  given  in  each  case  must,  of  course,  be  used  on  the 
motor.)  After  the  cells  have  been  "formed"  as  described  below,  they 
may  be  kept  charged  sufficiently  for  light,  intermittent  service  by  con- 
necting up  10  cells  of  gravity  battery  in  series  with  the  four  storage  cells, 
the  copper  terminal  of  the  blue-stone  battery  being  connected  to  the  pos- 
itive terminal  of  the  storage  battery.  This  connection  may  be  left  per- 
manently on,  during  ihe  use  of  the  storage  cells  as  well  as  when  they  are 
idle,  the  only  attention  necessary  being  the  replenishment  of  the  gravity 
cells  at  comparatively  long  intervals. 

The  plates  of  the  storage  cells  are  pasted,  the  positives  with  a  thick 
paste  made  of  red  lead  and  dilute  sulphuric  acid,  and  the  negatives  with  a 
similar  paste  made  of  litharge  and  dilute  acid.  The  acid  should  be  one- 
tenth  concentrated  sulphuric  acid  and  nine-tenths  water,  and  the  water 
should  be  distilled ;  the  proportions  of  one  and  nine  parts  are  by  weight, 
not  volume.  In  mixing,  always  pour  the  acid  into  the  water,  never  the 
reverse.  The  pastes  must  be  mixed  with  wooden  spatulas  in  glass  or 
earthenware  vessels,  and  should  be  so  thick  (containing  so  little  dilute 
acid)  as  to  appear  almos't  powdery.  The  pastes  are  applied  to  the  sides 
of  the  plates  and  pressed  firmly  in  with  the  spatulas  until  the  surface  of 
the  paste  is  flush  with  the  edges  of  the  little  shelves ;  the  entire  surfaces 
of  the  lead  strips,  except  the  edges  of  the  shelves,  must  be  covered  evenly. 
The  best  procedure  will  be  to  take  all  the  positive  plates  first ;  lay  them 
flat  on  a  board,  and  apply  red  lead  paste  to  one  side.  Set  them  aside 
and  mix  the  litharge  paste  (in  a  separate  vessel  and  with  a  separate  spat- 
ula), and  then  treat  one  side  of  all  the  negative  plates.  When  the  plates 
are  all  dry,  turn  them  over  and  treat  the  other  sides,  being  careful  not  to 
jar  out  the  paste  already  on  the  under  sides.  It  should  be  remembered, 


CONSTRUCTION  OF  A  SIMPLE  STORAGE  BATTERY.  213 

too,  that  two  of  the  negative  plates  in  each  cell  are  to  be  treated  on  one 
side  only — the  side  which  comes  next  to  the  neighboring  positive  plate. 

When  the  plates  are  all  pasted,  assemble  them  in  their  cells,  as  de- 
scribed above,  and  then  rivet  the  ends  of  the  connectors,  C  -f-  ,and  C  — , 
to  the  horizontal  terminal  strips,  T  and  T2.  Connect  the  positive  termi- 
nal of  one  cell  to  the  negative  terminal  of  its  neighbor,  and  fill  all  the 
cells  with  a  solution  consisting  of  one  part  concentrated  sulphuric  acid 
and  four  parts  distilled  water,  measuring  by  weight.  Connect  the  series 
of  cells  in  an  arc  light  circuit,  just  as  though  they  were  arc  lamps,  and  let 
ihe  current  pass  through  them  from  the  positive  to  the  negative  terminal 
of  the  series  until  the  paste  on  the  negative  plates  has  all  turned  color. 
The  cells  will  then  be  "formed"  and  ready  for  service.  They  should  not 
be  allowed  to  remain  charged  long  before  being  put  into  service,  and  it 
will  be  advisable,  therefore,  to  have  the  apparatus  for  which  they  are 
to  furnish  current  all  ready  to  start  up  before  putting  the  cells  in  circuit 
for  formation.  The  arc  light  circuit  on  which  the  cells  are  "formed"  may 
have  any  current  value  from  4  to  20,  but  as  most  of  the  circuits  in  this 
country  carry  either  6.8  amperes  or  9.6  amperes,  one  of  these  values  will 
doubtless  be  found  in  the  charging  circuit. 

If  the  circuit  is  an  intermittent  one  (does  not  run  constantly,  24  hours 
a  day),  care  must  be  observed  to  take  the  battery  out  of  circuit  as  soon  as 
the  current  is  of:  at  each  shut-down,  so  that  it  cannot  discharge  in  case 
the  line  is  closed  before  current  is  restored. 


CHAPTER  XXVI. 


CONSTRUCTION   OF   A   CONSTANT-POTENTIAL   ARC   LAMP. 


With  a  small  screw-cutting  lathe,  a  drill  chuck  and  a  few  drills  and 
other  small  tools,  any  mechanic  of  average  ability,  having  a  fair  knowl-' 
edge  of  electrical  apparatus,  can,  by  using  the  accompanying  sketches 
as  working  drawings,  make  a  reliable  and  efficient  arc  lamp  for  use  on  a 
no-volt  continuous-current  circuit,  in  series  with  a  resistance  coil  of  8 
ohms,  or  a  duplicate  lamp  and  a  resistance  coil  of  1^2  ohms,  preferably 
the  latter.  Fig.  240  shows  the  frame  of  the  lamp,  one-half  in  cross-sec- 
tion. A  is  the  top-plate;  JB  is  the  floor-plate;  <7,  C,  are  short  side-rods; 
J)y  Z>,  are  long  side-rods;  £,  the  yoke;  Fy  the  bottom  carbon  holder;  /,/ 
and  k,  are  insulating  washers  of  hard  fibre.  The  under  side  of  the  top- 
plate,  A,  is  shown  by  Fig.  241.  It  is  of  cast-iron  or  brass,  and  is  provided 
with  two  lugs,  d}  d,  $4  m-  m  diameter  and  i  in.  long,  drilled  and  tapped 
J£  in.  deep  to  take  %  in.  gas  pipe ;  a  lug,  e,  ^  in-  in  diameter  and  i  in. 
long,  drilled  and  tapped  to  take  a  5-32-in.  machine-screw,  and  a  flange 
around  the  outer  edge,  1-16  in.  thick  and  y2  in.  deep.  At  diametrically 
opposite  points,  two  pins,  x9  x,  are  set  in  the  flange;  these  are  of  i-i6-in. 
steel  wire,  y2  in.  long.  The  centers  of  the  lugs,  d,  d,  are  2^  ins.  from 
the  center  of  the  plate,  and  the  center  of  the  lug,  e,  is  2.y2  ins.  from  the 
center  of  the  plate.  On  the  upper  side  of  the  plate  is  a  neck  (see  Fig. 
140)  i%  ins.  in  diameter  outside,  and  standing  i  in.  above  the  upper  sur- 
face of  the  plate.  This  neck  is  bored  J^-in.  deep  and  tapped  to  fit  a  J^-in. 
gas  pipe;  below  this  bore,  a  hole,  11-16  in.  in  diameter  is  drilled  clear 
through  the  plate.  A  fibre  washer,  i,  shown  enlarged  in  Fig.  247,  is  fit- 
ted to  this  hole ;  the  larger  diameter  of  the  washer,  i,  must  be  such  as  to 
allow  it  to  slip  down  freely  in  the  threaded  neck,  and  the  smaller  diam- 
eter must  fit  snugly  the  n-i6-in.  hole  at  the  base  of  the  neck;  the  bore 
at  the  top  of  the  washer  is  2/g  in.,  and  the  recess  in  the  under  side  is  y2  in. 
in  diameter.  The  washer  is  }/\  in.  thick,  and  the  flange  is  ]/§  in.  thick. 

The  short  side-rods,  CC,  are  pieces  of  J^-in.  gas  pipe,  5^  ins.  long, 
threaded  on  the  outside  at  the  upper  ends  and  on  the  inside  at  the  lower 


CONSTRUCTION  OF  A  CONSTANT-POTENTIAL  ARC  LAMP      21 


ends;  the  long  side-rods,  DD,  are  similar  pieces  of  gas  pipe,  19^2  ins. 
long,  and  the  outside  thread  is  an  inch  long.  The  short  and  long  side- 
reds  are  held  together  by  a  steel  plug,  /,  J^  'ms'  l°ng>  threaded  the  whole 
length  to  correspond  with  the  thread  in  the  side-rods;  the  washers,  jjt 
shown  enlarged  in  Fig.  247,  are  interposed  to  insulate  the  floor-plate,  By 
from  the  side-rods  and  top-plate.  These  washers  are  ^4  m-  m  outer  di- 
ameter and  3/2  in.  diameter  at  the  neck;  the  bore  is  such  as  to  allow  the 
threaded  plug,  Z,  to  slip  through  without  having  to  screw  it;  the  flange  is 
}i  in.  thick,  and  the  total  thickness  is  ^  in. 

The  floor-plate,  D  (Figs.  240  and  242),  is  of  brass  or  iron,  5  15-16  ins. 
diameter,  J/s  in  thick,  and  has  a  round  lug,  c,  ^4  in-  in  diameter,  andi 


FIG.  240. — SEMI-SECTIONAL  SKETCH 
OF  FRAME. 


FIG.    241.  —  UNDKKSIDK   OF  TOP- PLATE. 


Kite. 


FIG.    242. — UPPER   STDK  OF  FLOOR-PLATE. 


~]/2  in.  high.  There  are  two  */2-in.  holes,  b,  b,  a  ^-in.  hole,  a,  in  the  center, 
and  a  hole  through  the  center  of  the  lug,  c,  threaded  for  a  3-i6-in.  screw. 
The  distances,  center  to  center,  are  23/s  ins.  from  a  to  each  b  and  i  %£  ins. 
from  a  to  c;  the  lug,  c,  is  90  degs.  from  the  holes,  b  b.  If  it  is  more  con- 
venient, the  floor-plate  may  be  cut  from  sheet  brass  and  the  lug,  c, 
-soldered  on,  or  screwed  in  and  riveted. 

The  yoke,  E  (Figs.  240  and  247),  may  be  cut  from  a  strip  of  brass  i% 
Ins.  wide  and  3-16  in.  thick ;  the  center  hole  is  ^  in.  and  the  others  l/2  in. 
in  diameter;  the  distances  are  2^  ins.  each  way,  from  center  to  center  of 


2l6 


ELECTRICAL  DESIGNS 


the  large  hole  and  each  of  the  smaller  ones,  corresponding  to  the  location 
of  the  lugs  on  A,  and  the  holes  through  B.  The  carbon  holder,  F,  is 
made  of  two  pieces  of  brass  tubing,  the  long  one  y2  in.  inside  diameter 
and  the  short  one  of  a  size  to  fit  snugly  over  the  long  tube ;  the  two  are 
sweated  together  and  riveted  as  a  precaution  against  the  loosening  of 
the  solder  by  the  heat  from  the  arc  when  the  carbons  are  almost  burned 
out.  The  long  piece  of  tubing  is  threaded  from  the  end  of  the  short 
piece  }i  in.  down,  and  turned  down  to  %  in.  outside  diameter  the  balance 
of  its  length  to  allow  the  nut,  g,  to  slip  up  to  the  beginning  of  the  thread, 
and  to  form  a  mandrel  for  the  globe  holder.  A  y%-'m.  pin,  i  in.  long, 
must  be  driven  horizontally  through  the  shank  of  the  carbon  holder. 
Yi  in.  below  the  thread,  after  the  nut  is  on ;  the  pin  holes  should  be  drilled 
exactly  across  the  center  of  the  tube,  so  that  when  the  pin  is  inserted  the 
ends  will  project  from  diametrically  opposite  sides  of  the  shank.  This 
pin  is  to  support  the  globe-holder,  as  will  be  explained  further  along. 
The  carbon-holder  is  3  ins.  long  over  all,  and  the  short  piece  of  tubing, 


FIG.    243. — ELEVATION  AND        FIG.    244. — ELEVATION  AND  FIG.  245. — PLAN  VIEW  OF 

PLAN  OF  ARMATURE.  PLAN  OF  BRASS  FRAME.  MECHANISM,  MINUS  CLUTCH. 

which  forms  a  sleeve  over  the  holder-tube,  is  I  in.  long.     A  set-screw,  h, 
serves  to  clamp  the  carbon  in  the  holder.     The  yoke  is  insulated  from 
the  side-rods  by  washers,  /  and  &;  the  former  was  described  above,  and* 
the  latter  is  a  plain,  flat  fibre  washer  %  in.  thick,  ^  m-  diameter,  with  a 
J/2-in.  hole ;  screws,  m,  m,  hold  the  yoke  to  the  side-rods. 

The  lamp  is  of  the  clutch  type  and  the  moving  parts  consist  of  a  mag- 
net-armature, a  clutch  and  a  carbon-carrying  rod.  The  magnet  is  a. 
straight,  round  bar,  with  a  single  coil ;  the  core  (K  in  Fig.  245)  is  ^4  in-  m 
diameter  and  3  ins.  long,  with  a  shoulder  54  m-  long  at  each  end,  the  di- 


CONSTRUCTION  OF  A  CONSTANT-POTENTIAL  ARC  LAMP       217 

ameter  there  being  ^  in.  The  core  is  provided  with  two  insulating  heads 
of  fibre  %  in  thick,  the  hole  in  which  is  a  tight  fit  on  the  reduced  ends 
of  the  core,  and  after  it  is  wound  it  is  mounted  between  two  brass  frames, 
N,  N  (Figs.  244  and  245).  Each  frame,  N,  has  a  y2  in.  hole  drilled  in  its 
base,  the  exact  location  of  which  may  be  found  by  reference  to  Fig.  244. 
These  frames  have  standards  to  which  is  pivoted  the  armature,  P  (Figs. 
243,  245  and  246).  The  thickness  of  the  metal  is  }£  in.  throughout.  The 
back  ends  of  the  frames,  A7",  Nf  are  held  down  by  a  cross-bar,  q  (Fig.  245), 
which  has  a  J/-in  hole  in  the  center  to  allow  the  lug,  r,  to  come  through. 
The  lug  is  threaded  on  the  outside  to  take  a  nut  to  hold  down  the  cross- 
bar, q.  The  bar  is  y§  in.  thick,  ^4  m-  wide  and  a  trifle  over  2%  ins.  long 
so  that  the  ends  may  be  filed  to  fit  exactly  between  the  pivot  standards  of 
the  frames,  Nt  N. 

The  armature  is  a  piece  of  flat  Norway  iron,  3-16  in.  thick,  ^  in. 
wide  and  Iij4  ins.  long,  bent  into  a  U,  as  shown  in  Fig.  243,  and  provided 
with  two  ears,  r,  r,  y&  in.  thick,  which  carry  a  round  rod,  s,  of  steel, 


:G—  n^—Q) 


FIG.  246. — ELEVATION  OF  MECH- 
AXISM,    COMPLETE. 


Am.Ehc. 
FIG.  248. — CROSS-SECTION  OF  GLOBE-HOLDER. 


3-16  in.  diameter.  At  the  back  end  a  clip,  t,  of  brass,  is  riveted  on ;  this 
piece  has  a  hole  through  it,  tapped  for -a  3~i6-in.  machine  screw.  Pivot - 
holes,  whose  centers  coincide  with  the  dotted  line,  p,  p,  are  drilled  in  the 
sides  of  the  armature.  The  dimensions  specified  in  the  drawing  must  be 
carefully  observed. 

The  clutch  H  (Fig.  247)  is  a  flat  piece  of  brass  y&  in.  thick,  %  in. 
wide,  and  2^4  ins.  long,  with  a  7~i6-in.  hole  drilled  J^  in.  from  one  end 
and  a  J^-in.  slot,  y2  in.  deep,  sawed  in  the  other  end.  The  edges  of  the 
hole  must  be  very  slightly  rounded  to  prevent  the  clutch  from  cutting 


2i8  ELECTRICAL   DESIGNS 

into  the  carbon  rod ;  v  is  a  regulating  screw  to  trip  the  clutch ;  it  is 
Ins.  long  over  all,  3-16  in.  diameter  at  the  threaded  part,  and  j/s  in.  diam- 
eter beyond  the  shoulder.  The  shoulder  is  one  inch  from  the  end.  The 
armature,  frames,  clutch  and  carbon  rod  are  shown  assembled  in  Fig. 
246.  The  spring,  y,  which  pulls  the  armature  upward,  is  adjusted  by 
means  of  the  screw,  w,  which  screws  into  the  lug,  c,  on  the  top-plate. 
The  screw  is  5-32  in.  diameter  and  the  threaded  end  is  an  inch  long. 
The  play  of  the  armature  is  limited  by  the  back-screw,  s,  by  means  of 
which  the  length  of  arc  first  struck  is  adjusted;  the  screw,  vt  and  the 
spring,  y,  regulate  the  length  of  arc  while  burning.  The  spring  is  at- 
tached to  a  stout  brass  wire  strung  from  limb  to  limb  of  the  armature. 

The  magnet  is  wound  with  No.  30  double  cotton-covered  magnet 
wire,  32  layers  deep,  and  125  turns  long,  the  starting  end  being  connect- 
ed with  the  core  and  the  outer  end  with  the  bottom  carbon  holder.  Bind- 
ing posts  may  be  put  on  if  desired,  but  the  writer  prefers  to  carry  a  piece 
of  No.  12  rubber-covered  and  braided  wire  from  the  yoke  up  alongside 
one  of  the  side-rods,  making  this  the  negative  lamp  terminal ;  the  outer 
end  of  the  magnet  coil  may  be  connected  to  this  terminal  by  means  of  a 
piece  of  stout  wire  brought  through  the  floor-plate,  B,  of  the  lamp,  the 
hole  being  bushed  with  insulation,  and  the  positive  terminal  may  be  a 
binding  post  screwed  in  the  floor-plate  (which  is  in  electrical  contact  with 
the  carbon  rod). 

The  case  of  the  lamp  is  a  piece  of  thin  sheet  brass,  6  ins.  X  19  ins., 
"bent  into  a  6-in.  tube  and  riveted  at  the  lap ;  at  one  end  of  the  tube  thus 
formed  and  diametrically  opposite  each  other,  are  bayonet  slots  which  en- 
gage with  two  pins  projecting  inwardly  from  the  flange  of  the  top  plate  of 
the  lamp.  The  carbon  rod  is  a  piece  of  brass  tubing  j£-in.  diameter  out- 
side, 24  ins.  long,  with  a  i-i6-in.  wall.  The  upper  carbon  holder  can  be 
purchased  for  a  small  sum  and  is  not  worth  the  trouble  of  making.  The 
"ball  and  shank  must  be  made  to  fit  the  carbon-holder  and  the  bore  of  the 
carbon  rod ;  the  shank  should  be  an  inch  long,  very  slightlty  tapered.  Drill 
a  1-16  in.  hole  clear  through  the  rod,  ^  in.  from  the  end,  before  inserting 
Ihe  shank ;  then  tin  the  shank  and  the  inside  of  the  end  of  the  carbon  rod, 
drive  the  shank  in,  and  solder  through  the  holes. 

The  globe-holder  (Fig.  248)  consists  of  a  disc  of  brass  (or  iron) 
3-16  in.  thick  and  5  ins.  in  diameter,  having  a  piece  of  brass  tubing 
screwed  into  its  center  and  three  lips  riveted  at  equidistant  points  around 
the  edge.  The  tube  in  the  center  must  fit  snugly  over  the  shank  of  the 
carbon-holder  (Fig.  240)  and  it  has  two  bayonet  slots  at  the  upper  end 
which  fit  over  the  ends  of  the  pin  driven  transversely  through  the  carbon- 


CONSTRUCTION.  OF  A  CONSTANT-POTENTIAL  ARC    LAMP      219 

holder  shank.  This  tube  must  measure  ij^  ins.  long  above  the  disc. 
The  ears  are  simple  brass  strips  each  J^  in.  thick,  y2  in.  wide  and  2  ins. 
long,  with  y2  in.  of  its  length  bent  up  almost  at  right  angles  to  the  bal- 
ance ;  3-16  in.  thumb-screws  in  the  up-turned  lips  serve  to  hold  the  globe 
by  its  rim. 

Two  arc  lamps  such  as  the  one  above  described  will  work  together, 
in  series  with  a  resistance  coil  of  ij^  ohms,  on  any  no-volt  direct- 
current  circuit. 


CHAPTER  XXVII. 


AN  EXPERIMENTAL  NERNST  LAMP. 


•This  lamp,  invented  by  the  physicist,  Nernst,  of  Gottingen,  consists 
of  a  rod  of  dense  magnesia  with  platinum  terminals.  This  rod  is  con- 
nected in  series  with  a  dead  resistance,  and  an  e.  m.  f.  (preferably  alter- 
nating) of  from  200  to  600  volts  is  applied  to  the  arrangement.  Upon 
heating  the  magnesia  rod,  by  a  blow  pipe,  for  example,  it  becomes  a  con- 
ductor and  passes  sufficient  current  to  raise  its  temperature  to  that  of  in- 
tense incandescence.  In  the  more  recent  types  Nernst  uses  a  large  pro- 
portion of  thoria  in  the  rod. 

An  increase  of  current  in  the  lamp  causes  a  rise  in  its  temperature 
and  a  drop  in  its  resistance  and,  at  the  temperature  at  which  the  lamp  is 
used,  this  drop  in  resistance  is  so  great  that  considerably  less  e.  m.  f.  is 
required  to  push  the  increased  current  through  the  rod,  so  that  the  lamp 
is  unstable,  and  without  the  dead  resistance  the  lamp  would  be  destroyed 
by  the  excessive  current  that  would  flow  through  it.  The  efficiency  yof 
the  lamp,  according  to  tests  made  abroad,  is  about  1.5  watts  per  candle- 
power,  including  the  watts  lost  in  the  dead  resistance.  The  lamp  gives 
a  beautiful  and  pleasant  white  light  and  its  life  is  claimed  to  be  very  great. 

A  number  of  these  lamps  have  been  constructed  at  the  physical 
laboratory  in  Bethlehem,  Pa.,  by  Prof.  W.  S.  Franklin  and  Mr.  R.  B. 
"Williamson.  After  many  trials  the  following  procedure  was  found  to 
give  good  results :  A  mixture  of  calcined  magnesium  oxide  (composition 
of  mixture  given  below)  is  tamped  as  compactly  as  possible  into  a  smooth 
bore  brass  tube  lined  with  two  or  three  thicknesses  of  stiff  writing  paper. 
This  paper  should  be  fixed  in  place  with  a  little  glue  and  baked  dry.  The 
tube  full  of  magnesia  is  then  slowly  baked  on  a  metal  plate  over  a  Bunsen 
burner  until  the  paper  is  completely  charred,  when  the  magnesia  rod  may 
be  pushed  out.  The  rod  is  then  calcined  before  a  blow  pipe,  heating  it 
slowly  and  uniformly  to  avoid  cracking  by  unequal  shrinkage.  The  rod 
is  then  broken  to  a  length  of  about  2^/2  ins.  and  laid  upon  a  bed  of  mag- 


EXPERIMENTAL  NERNST  LAMP 


221 


nesia.  Two  ordinary  arc  carbons  are  brought  up  to  the  ends  of  the  rod, 
one  carbon  being  fixed  by  weights,  the  other  being  preferably  held  in  the 
hand.  Several  hundred  volts  e.  m.  f.  are  applied  to  the  carbons  with 
dead  resistance  in  circuit  and  the  magnesia  rod  is  heated  by  the  blow  pipe 
until  the  current  starts.  As  the  magnesia  rod  rises  in  temperature  it 
shrinks  greatly,  and  it  must  be  subjected  to  very  slight  end  pressure  to 
prevent  the  formation  of  cross  cracks ;  too  much  pressure  will  cause  lon- 
gitudinal cracks.  The  current  is  then  increased  until  the  magnesia  rod 
becomes  slightly  soft,  when  it  may  be  straightened  if,  as  is  likely,  it  has 
curled  up  in  shrinking.  The  rod  is  then  allowed  to  cool  and  ground  on 
an  emery  wheel  to  the  required  shape,  as  described  below. 

The  most  convenient  source  of  current  for  the  purpose  of  this  pre- 
liminary heating  and  for  operating  the  finished  lamp  is  a  step-up  trans- 
former with  a  rheostat  in  the  primary  circuit ;  a  secondary  e.  m.  f.  of  1,000 
volts  is  satisfactory.  This  e.  m.  f.,  of  course,  falls  off  greatly  when  the 
current  starts,  because  of  the  action  of  the  primary  rheostat. 

The  magnesia  mixture  may  be  pure 
calcined  magnesia  with  a  slight  amount  of 
magnesium  chloride  ground  up  with  it  to  serve 
as  a  bond.  A  slight  amount  of  soluble  silicate 
of  soda  is  also  a  good  bond.  The  mixture 
should  be  only  moist  enough  to  pack  like 
flour  ;  it  is  better  to  have  it  perfectly  dry  than 
too  moist.  A  lamp  made  of  pure  magnesia  or 
of  magnesia  with  i  per  cent  or  less  of  powder- 
ed silica,  has  a  very  high  resistance  and  can 
scarcely  be  started  with  less  than  1,000  volts, 
and  then  with  difficulty.  After  it  is  once 
started,  however,  the  resistance  falls  so  that 
even  a  pure  magnesia  rod  will  operate  with, 
say,  300  volts  per  inch  of  length.  A  lamp 
which  is  very  much  easier  to  start  is  made  by 
mixing  from  2  to  6  per  cent  of  pounded  glass 

with  the  powdered  magnesia.     Perhaps  a  lime  glass  would  be  best  for 
this  purpose. 

The  magnesia  rod  should  be  about  i  in.  or  ij^  ins.  in  length,  and 
about  J/s-in.  in  diameter,  with  slightly  enlarged  grooved  ends :  platinum 
wire  is  wound  two  or  three  times  around  these  ends  and  covered  with  a 
paste  of  magnesia,  pounded  glass,  and  water  glass  (or  simply  water).  The 
lamp  is  conveniently  mounted  by  binding  the  platinum  wires  to  the  side 
of  a  small  glass  tube.  Fig.  249  shows  the  finished  lamp  full  size. 


FIG.    249. — A  NERNST  LAMP. 


222  ELECTRICAL   DESIGNS 

A  lamp  made  as  above  described,  with  about  I  per  cent,  of  pounded 
glass  and  I  per  cent,  of  powdered  silica,  the  rod  being  about  1)4  ins.  long- 
and  J^-in.  in  diameter,  operated  on  250  volts  (between  platinum  termi- 
nals), takes  0.8  ampere,  and  gives  fully  175  candle-power,  although  the 
candle-power  has  not  been  measured  at  Bethlehem.  It  has  been  found 
that  the  silicates  of  sodium  and  potassium  (or  perhaps  simply  the  sodium 
and  potassium)  are  slowly  expelled  by  the  heat  while  the  lamp  is  in  use, 
causing  the  resistance  to  become  slowly  greater. 

Commercial  magnesia  (calcined  Grecian  magnesite)  makes  good 
lamps  without  any  admixture  of  silica,  although  its  resistance  is  rather 
high  unless  it  is  mixed  with  powdered  glass. 

An  attempt  was  made  to  fuse  magnesia  into  a  compact  mass  in  an 
electric  furnace  (100  amperes  at  about  90  volts),  but  it  was  found  that  the 
boiling  point  of  magnesia  (at  atmospheric  pressure)  is  about  the  same 
as  its  melting  point,  so  that  the  material  vaporized  about  as  rapidly  as  it 
was  melted.  The  operation  would,  no  doubt,  succeed  under  pressure. 
During  this  work  with  the  electric  furnace  it  was  necessary  to  keep  a 
close  watch  of  the  action,  and  a  small  piece  of  heavily  smoked  glass  was 
used  to  screen  the  eyes,  leaving  the  forehead  exposed,  and  a  sever  case  of 
sunburn  was  produced,  although  the  heat  on  the  face  was  not  excessive. 

A  most  striking  experiment  is  to  mount  a  glass  tube  as  a  Nernst 
lamp.  A  large,  thin  walled  tube  gives  the  best  effect.  Wind  copper  wire 
terminals  about  4  ins.  apart  on  a  thin  walled  glass  tube  3^-in.  or  J^-in. 
in  diameter.  Connect  to  the  secondary  of  a  step-up  transformer  with  a 
rheostat  in  the  primary.  Heat  the  tube  along  one  side.  The  current 
starts  along  a  narrow  strip  of  the  glass,  heats  it  to  bright  redness,  and 
this  heated  strip  gradually  widens  until  the  whole  tube  is  melted  dowti 
This  experiment  was  tried  in  Bethlehem  with  a  i,ooo-volt  secondary,  but 
it  would  certainly  be  possible  to  perform  the  experiment  successfully 
with  as  low  an  e.  m.  f.  as  100  volts,  and  direct  current  would  answer  as 
well  as  alternating.  With  low  e.  m.  f.  the  distance  between  the  copper 
terminals  should  be  much  less  than  4  ins.,  and,  of  course,  a  rheostat 
should  be  included  in  the  circuit. 


CHAPTER  XXVIII. 


CONSTRUCTION   OF  AN   INDUCTION  COII,. 


Since  the  advent  of  the  Rontgen  discovery  the  induction  coil  ha$ 
risen  to  a  much  more  prominent  place  as  a  scientific  and  practical  instru- 
ment. It  has  very  naturally  been  greatly  improved  in  construction  with- 
in the  past  year,  but  inasmuch  as  these  improvements  are  not  generally 
known  and  used,  the  writer  has  presumed  to  believe  that  a  description 
of  them  may  be  interesting. 

The  basis  of  the  discussion  will  be  the  construction  of  a  6-inch  spark 
coil,  but  it  may  be  profitably  remembered  that  the  average  induction  coil 
built  in  sections  may  be, thus  rebuilt,  and  oftentimes  the  length  of  spark 
it  is  capable  of  giving  thereby  trebled,  even  though  thirty  or  forty  per 
cent,  of  the  secondary  is  removed  in  order  to  accomplish  the  construction. 

Many  modern  coils  are  built  on  lines  that  make  extensive  internal 
leakage  a  great  possibility.  Some  coils  are  made  with  as  much  as  twen- 
ty-five pounds  of  wire  in  the  secondary,  and  yet  under  the  most  favorable 
conditions  the  spark  obtained  is  but  six  inches  in  length.  The  makers  of 
such  coils  broadly  claim  that  it  is  impossible  to  break  down  the  insulation 
of  their  apparatus,  but  in  view  of  the  fact  that  a  6-inch  coil  can  be  made 
with  a  5-pound  secondary,  it  is  easy  to  see  that  the  coils  just  referred  to 
are  broken  down  already,  and  that  it  is  a  case  of  spoiling  a  bad  eg^j — a 
manifest  impossibility. 

The  principal  leak  in  an  induction  coil  is  from  the  secondary  to  the 
primary,  as  is  shown  in  Fig.  250.  Between  the  points  of  leakage  indi- 
cated the  full  difference  of  potential  of  the  coil  exists.  The  %-in.  of  hard 
rubber  and  the  almost  negligible  air  gap  usually  provided  can  scarcely  be 
expected  to  withstand  the  e.  m.  f.  that  will  urge  a  discharge  across  a  six- 
inch  air  gap. 

A  second  source  of  leakage  is  shown  in  Fig.  2 50- A,  and  exists  at  the 
separator  pieces  between  sections.  The  insulation  between  the  primary 
and  secondary  is  broken  in  its  continuity  by  these  pieces,  and  as  it  is  im- 
possible to  make  an  electrically  tight  joint,  such  insulation  as  is  provided 


224 


ELECTRICAL  DESIGNS 


is  no  more  effective  than  an  equivalent  gap  of  air.  The  insulation  between 
primary  and  secondary  must  be  a  continuous  homogeneous  mass,  and 
sufficiently  thick  to  withstand  the  maximum  e.  m.  f.  of  the  coil.  Fig.  251 
illustrates  the  method  of  insulating  a  secondary  section.  The  spaces,  5  S, 
are  to  be  filled  with  paraffine  or  some  equivalent  continuous  insulator. 

Covered  wire  for  an  induction  coil  is  not  necessary,  and  the  use  of 
silk  wire  is  a  most  expensive  construction,  from  which  absolutely  no  ad- 


FIG.  250. — SHOWING  LEAKAGE  FROM 
SECONDARY. 


MMMMMM&MM 

Secondary 


Insulation 


\ 


Am.Elee. 
FIG.    250A. — LEAKAGE  THROUGH  JOINT. 


FIG.    252. 


Secondary 


J                            Primary  and  Core                         £ 

S 

Am.EUc* 

S 

S 

Secondary 

S 

Am.Ekc. 
FIG.  253. — DIAGRAM  OF  COIL  CONNECTIONS. 


FIG.  251. — IDEAL  INSULATION  FOR 
SECONDARY. 


vantage  can  be  gained.  One  way  is  to  use  bare  wire,  winding  a  thread 
between  adjacent  turns,  as  shown  in  Fig.  252.  Colored  thread  should  be 
avoided.  The  space  between  the  layers  should  be  at  least  four  or  five 
times  the  thickness  of  the  insulation  between  the  turns.  The  insulation 
between  the  turns  of  an  induction  coil  is  about  5  mils  (.005  in.)  thick,  and 


CONSTRUCTION  OF  AN  INDUCTION  COIL  225 

experience  has  shown  that  this  is  none  too  much.  A  space  of  i-64-in. 
can  be  used  between  the  layers  to  advantage.  This  space  should  be 
filled  with  absorbent  paper  that  will  readily  soak  up  paraffine  wax. 

rig.  254  shows  a  regular  sectioned  dimension  drawing  of  the  6-in. 
spark  coil  already  referred  to.  It  would  be  idle,  to  enter  into  a  long  dis- 
sertation on  the  various  features  of  this  coil  that  are  common  to  every  in- 
strument of  a  similar  nature,  and  only  the  novel  ones  will  be  discussed 
and  the  quantitative  measurements  given.  The  secondary  coils  are  con- 
structed of  bare  wire,  absorbent  paper  and  cotton  thread,  substantially  as 
indicated  heretofore.  Care  must  be  taken  in  the  winding  to  keep  away 
at  least  54~m-  with  the  wire  from  the  edge  of  the  paper  layer,  partly  for 
the  added  insulation  between  the  layers  and  partly  to  prevent  the  annoy- 
ance of  the  end  turn  slipping  out  when  handling  the  section.  If  an  old 
coil  is  being  rebuilt,  it  will  not  pay  to  thus  rewind  it.  Sufficient  wire  from 
the  inside  of  the  secondary  sections  should  be  removed  to  admit  of  reas- 
sembling it  as  per  drawing,  a  comparatively  easy  thing  to  do,  and  the 
results  will  be  nearly  as  good  as  with  the  coil  here  described. 

The  great  feature  of  the  coil  is  the  method  of  supporting  and  insulat- 
ing its  primary  and  secondary.  A  long  box  is  constructed  as  per  draw- 
ing, and  from  the  geometrical  center  of  the  ends  is  supported  the  tube 
that  forms  the  enclosing  envelope  for  the  primary  coil  and  its  core.  The 
secondary  coil  is  divided  into  six  sections,  each  supported  on  a  piece  of 
hard  rubber  tube  with  end  collars  of  glass  or  hard  rubber.  This  hard 
rubber  tube  allows  j/2-in.  in  the  clear  between  its  interior  surface  and  the 
primary  envelope.  The  glass  collars  are  square,  and  are  of  such  a  shape 
that  they  just  fit  the  inside  of  the  box,  and  in  their  lateral  dimensions  are 
a  perfect  measure  of  its  interior  section.  The  space  between  adjacent 
sections  is  %-m.,  and  between  the  last  coils  and  the  end  pieces,  5/2-in.  is 
provided.  The  coil  is  wound  to  a  diameter  of  6  ins.,  the  internal  dimen- 
sions of  the  box  surrounding  it  being  8  ins.  square. 

Before  assembling,  the  coils  are  boiled  for  a  long  time  in  paraffine, 
and  are  removed  therefrom  only  when  the  wax  has  cooled  sufficiently  to 
attain  a  mushy  consistency.  They  are  preferably  assembled  while  in  this 
state,  for  large  soft  clots  of  wax  adhere  to  the  coils  and  close  in  on  the 
bobbin  on  which  the  coil  is  put,  thus  filling  up  objectionable  air  spaces. 
The  assembling  of  the  coil  being  complete,  each  secondary  will  be  mount- 
ed on  a  tube  in  the  box  and  will  rest  in  a  partition  made  on  two  sides  of 
glass  or  hard  rubber.  Nowhere  will  any  secondary  section  have  any  con- 
nection with  any  primary  section  except  through  paraffine  wax  in  a  con- 
tinuous mass  that  cannot  be  broken  down  unless  penetrated.  The  great 


CONSTRUCTION  OF  AN  INDUCTION  COIL 


227 


merit  is  the  continuity  of  the  insulation  and  the  entire  absence  of  joints. 
To  attain  this  result,  the  box  must  be  filled  with  boiling  paraffme  at  all 
partitions,  thus  filling  up  all  the  air  spaces,  of  course,  first  making  the 
proper  connections.  The  top  of  the  box  is  then  put  on  and  the  paraffine 
is  allowied  to  set.  In  setting  is  will  shrink  a  certain  amount,  and  this 
space  must  be  filled  with  more  paraffine. 

The  coil  is  to  be  mounted  on  a  box  containing  the  condenser  in  the 
usual  way.  It  will  be  well  to  divide  the  condenser  into  sections,  as  shown 
in  the  diagrammatic  connections  of  Fig.  253.  If  the  coil  is  to  excite  a 


FIG.    255. — PERSPECTIVE  OF  VIBRATOR, 

Crookes'  tube,  this  in  an  important  matter.  Some  tubes  that  are  capable 
of  giving  admirable  results  often  signally  fail  to  do  so  on  a  coil  of  great 
capacity,  but  will  operate  perfectly  on  a  smaller  one.  The  reason  of  this 
is  found  in  the  fact  that  the  large  coil  may  not  be  in  as  close  resonance 
with  the  tube  as  the  smaller  one.  By  the  use  of  the  variable  condenser, 
the  resonance  of  the  coil  can  be  varied  in  pitch  and  its  range  of  excitation 
of  tubes  widened  materially.  The  principal  dimensions  of  the  coil  just 
described  are  as  follows : 

Primary  coil.    Two  layers  of  No.  12  B.  &  S.  wire,  single  cotton  cov- 


228  ELECTRICAL   DESIGNS 

ered,  wound  on  a  fibre  tube  and  surrounded  with  a  hard  rubber  envel- 
oping tube,  as  per  drawing1. 

Secondary  coil.  Five  pounds  of  No.  36  B.  &  S.  bare  wire,  wound  in 
six  sections,  as  shown  and  described. 

Support.  A  mahogany  box  supporting  primary  envelope,  and  glass 
partitions,  as  described  and  shown. 

Condenser.  Seventy-five  sheets  of  tin-foil  7  ins.  X  9  ins-  alternated 
with  sheets  of  paraffined  paper  8  ins.  X  10  ins. 

A  word  about  the  secondary  connections  may  not  be  out  of  place 
because  of  the  confusion  that  has  arisen  among  amateurs.  It  is  custom- 
ary to  wind  the  secondary  coils  exactly  alike  with  the  outer  lead  on  one 
flat  face  and  the  inner  lead  on  the  other.  If  such  similar  coils  are  slipped 
on  the  core  in  the  same  way,  it  will  be  necessary  in  order  to  connect  them 
in  series  to  join  the  inner  end  of  one  to  the  outer  one  of  its  next  neighbor. 
This  will  require  that  the  connecting  wire  must  be  brought  up  between 
sections  and  in  this  position  it  will  be  very  difficult  to  insulate.  There- 
fore the  coils  are  slipped  on  in  alternate  reverse  order.  By  this  is  meant 
that  if  the  first  coil  is  put  on  in  one  direction,  the  next  is  put  on  so  that 
similar  ends  face  each  other.  To  connect  the  coils  so  placed  in  series,  the 
like  ends  must  be  connected.  A  moment's  inspection  of  this  connection 
will  show  that  the" -current  travels  about  the  core  in  the  same  direction 
through  all  bobbins,  and  that  the  arrangement  does  not  connect  the  bob- 
bins in  opposition,  as  has  been  popularly  supposed. 

The  circuit  breaker  or  interrupter  is  one  of  the  most  important  parts 
of  the  coil  and  little  has  been  done  to  improve  it.  The  ordinary  vibrator 
is  perhaps  the  most  convenient  automatic  circuit  breaker,  but  it  is  very 
defective  in  many  respects.  One  of  its  chief  faults  is  that  it  keeps  the 
circuit  open  too  long  and  closed  for  so  short  a  period  that  the  core  does 
not  have  time  to  fully  charge  or  the  current  to  attain  its  full  value.  An 
interesting  modification  that  tends  to  achieve  this  result  is  shown  in  Fig. 
255  and  is  drawn  in  suitable  form  to  apply  to  the  coil  just  discussed.  Its 
principle  is  as  follows :  The  spring,  C,  presses  tightly  against  its  contact, 
Kt  at  all  times  except  when  it  is  struck  by  the  hammer  of  the  vibrator, 
when  contact  is  broken  for  an  instant.  Thus  the  break  is  instantaneous 
and  the  circuit  is  closed  for  a  definite  period  cf  time.  The  other  screws 
are  to  limit  the  motion  and  frequency  of  vibration  cf  the  hammer. 

As  indicated  in  the  illustration,  a  double-pole  switch  and  a  means  of 
varying  the  condenser  are  to  be  placed  on  this  induction  coil.  A  double- 
pole  double-throw  baby  knife  switch  is  the  most  suitable  for  the  reversing 
device,  and  for  the  condenser  a  pair  of  plugs  and  plates  will  be  found 


CONSTRUCTION  OF  AN  INDUCTION  COIL  229 

convenient.  These  are  not  shown  on  the  drawing  because  they  would 
tend  to  confuse  the  more  important  details  of  the  vibrator.  It  is  obvious 
that  they  should  be  placed  in  a  convenient  and  symmetrical  position  and 
that  further  mention  of  them  would  be  more  perfunctory  than  interesting. 
It  will  be  noted  that  this  coil  is  designed  on  lines  that  seem  to  direct- 
ly defy  all  laws  of  magnetic  efficiency  with  regard  to  the  distance  between 
primary  and  secondary.  Many  might  hesitate  before  spending  their 
time  and  money  on  such  a  construction.  The  reader  is  assured  that  the 
dimensions  herein  given  are  the  result  of  a  series  of  progressive  experi- 
ments, and  each  coil  in  the  series  was  constructed  with  the  idea  of  im- 
proving the  last.  Not  until  the  liberal  insulation  shown  was  adopted 
were  maximum  results  obtained  and  even  now  the  advisability  of  carry- 
ing the  principle  further  is  being  considered.  The  smaller  amount  of 
wire  and  its  inferior  magnetic  position  are  more  than  compensated  by  the 
absence  of  leakage  and,  moreover,  the  extremely  low  internal  resistance 
of  such  a  coil  enables  it  to  produce  a  much  more  highly  calorific  spark 
or  as  it  is  commonly  termed,  a  fatter  one,  than  if  the  older  and  more 
conventional  construction  were  followed. 


CHAPTER  XXIX. 


CONSTRUCTION  OF  A  TKSLA-THOMSON  HIGH   FREQUENCY  COII,. 


The  following  is  a  description  of  the  construction  of  a  Tesla-Thom- 
son  high  frequency  coil,  large  enough  to  give  a  five-inch  spark  and  ex- 
cite Rontgen  ray  tubes. 

To  excite  the  Tesla-Thomson  coil,  a  high  potential  transformer  of 
from  10,000  to  15,000  is  necessary.  The  construction  of  this  transformer 
will  be  first  given.  Fig.  256  gives  a  partial  cross-section  of  the  trans- 
former, which  is  made  as  follows :  A  two-inch  iron  pipe,  sixteen  inches 
long,  is  slotted  the  whole  length,  either  in  a  milling  machine,  planer  or 
shaper.  This  slot  need  not  be  more  than  i-i6-in.  in  width.  The  pipe 


PIPE  SLOTTED  AND  FILLED  WITH  SMALL  SOFT  IRON  WIRE 


PAPER  INSULATI 
PRIMARY  WINDINQ*13  B.  4  3. 


FIBRED  INSULATION 


FIG.    256. — HIGH  TENSION  TRANSFORMER. 


is  then  insulated  with  ordinary  wrapping  paper  to  an  outside  diameter 
of  2,y$  ins.,  shellac  being  freely  used,  and  is  then  wound  with  No.  13  3. 
&.S.  double  cotton-covered  wire  for  its  whole  length  (one  layer).  It  is 
then  covered  with  paper  and  shellacked  until  the  outside  diameter  is  2^£ 
ins. 

The  next  step  is  to  fill  the  pipe  with  soft  iron  wires,  No.  16  B.  &  S.,^ 
each  wire  being  cut  eighteen  inches  long.  This  completes  the  primary 
winding  of  the  high  tension  transformer. 

The  secondary  winding  of  this  transformer  consists  of  ten  roils  wound 


TESLA-THOMSON  HIGH  FREQUENCY  COIL 


in  a  form  and  thoroughly  taped  and  insulated.  This  form  is  shown  in 
Fig.  257  and  can  be  easily  made  cf  wood.  The  wire  is  wound  in  Ihis 
form,  shellacked,  removed,  taped  and  baked.  These  ceils  are  then  slipped 
over  the  primary  winding,  between  each  coil  being  placed  a  disc  of  card- 
board J<j-in.  thick,  care  being  taken  to  connect  the  coils  so  that  none  will 
be  in  opposition. 

The  spark  gap  (see  Fig.  258)  is  made  as  shown  in  diagram.     The 


NO.  2  B.  &S.  COPPER  WIRE, 


HARD  RUBBER  TUBE^~ 


WOOD  RODS' 


FIG.    257. — FORM   FOR  WINDING   COILS 


WOOD  BASE 

FIG.    258. — SPARK   GAP. 


SECONDARY  HIGH  TENSION  TRANSFORMER 

AAAAAAAAAAAAAAAAAAAAAA/V\AAAAAA^^AMA/VVWS 


Tlft  FOIL 
8'xlO* 


GLASS  10  x  12 


PRIMARY  HIGH  FREQUENCY  COIL 

VvVWWWWWVWVW\J 


/VVV/yAA/VVAAAAAAAAAAAAA/NAAAAAMAAA/WSAM^ 
SECONDARY  HIGH  FREQUENCY  COIL 


oo 


FIG.   259. — CONDENSER  PLATE. 


FIG.    26l.— DIAGRAM  OP  CONNECTIONS. 


copper  wires  fit  rather  close  in  the  holes  drilled  through  the  hard  rubber 
tubing,  so  that  the  length  of  gap  can  be  adjusted  with  ease. 

The  condenser  is  made  of  ordinary  loin.  X  12-in.  window  glass.  A 
sheet  of  tin  foil  8  ins.  X  10  ins.  is  pasted  on  one  side  of  the  glass  with 
shellac,  leaving  a  margin  of  one  inch.  (See  Fig.  259.)  A  strip  of  tin 


ELECTRICAL  DESIGNS 

foil  two  inches  wide  is  placed  across  one  corner,  this  strip  being  placed 
alternately  on  each  side.  For  each  side  of  this  condenser  there  should 
be  fifteen  plates. 

To  build  this  condenser  proceed  as  follows :  Place  on  a  smooth  sur- 
face a  condenser  plate  with  the  connecting  strip  projecting  on  the  right. 
On  top  of  this  plate  place  another  piece  of  glass,  10  ins.  X  12  ins.,  that 
has  no  tin  foil  on  it  at  all.  Then  place  a  condenser  plate  with  the  strip 
projecting  on  the  left.  Then  a  piece  of  glass  without  tin  foil,  and  on  top 
of  this  a  condenser  plate  with  the  strip  projecting  on  the  right,  and  so  on. 
This  construction  gives  two  thicknesses  of  glass  between  each  sheet  of 
tin  foil,  which  is  absolutely  necessary. 

H -18- »i 

~Tfi 300^COOOOOOOQOOOOOOOOOOOOOOOOOOOOOOOOOGOOoooooooooo0oooOOOOOOOOOOOOOOOOOOOCr. 
^^VDDIUADV  r»rtii     entto^o  a     o.  o      IA/IOC-O   i*.i    n*n 


COIL  FOUR8  B.  &  S.  WIRES  IN  PARALLEL 


>*8ECONDARY  C0.ll.   31  B,  &  3. 


.i-LoUCOOQUOOOOOOOOOOOOQOOQOOOOOQOoooOQO  - ; ; 

FIG.   260. — PRIMARY  AND  SECONDARY  COILS  OF  HIGH  FREQUENCY  TRANSFORMER. 

The  high  frequency  coil  is  made  as  follows :  Wind  an  8-inch  paper 
cylinder  eighteen  inches  long  with  No.  31  B.  &  S.  double  cotton-covered 
wire  (or  larger),  leaving  a  margin  at  each  end  of  about  one  inch.  This 
is  the  secondary  winding.  The  primary  winding  is  placed  on  a  1 2-inch 
paper  cylinder  eighteen  inches  long  and  consists  of  fourteen  turns  of  four 
No.  8  B.  &  S.  double  cotton-covered  wires  in  parallel.  Each  of  these  No. 
8  wires  is  wound  on  separately,  then  the  four  ends  at  the  beginning  and 
ending  are  soldered  together.  Between  wires  of  different  polarity,  as 
an  extra  precaution,  two  turns  of  cord  are  wound.  The  primary  and  sec- 


TESLA-THOMSON  HIGH  FREQUENCY  COIL  233 

ondary  coils  are  then  shellacked  and  baked.  After  being,  baked,  the  sec- 
ondary coil  is  placed  concentrically  (see  Fig.  260)  inside  the  .primary  and 
the  connections  as  shown  in  Fig.  261  then  made. 

The  primary  of  the  high  tension  transformer  must  be  excited  with  an 
alternating  current.  With  a  frequency  of  60  cycles  per  second,  50  volts 
will  suffice,  and  for  125  cycles  per  second  100  volts.  The  length  of  the* 
spark  from  the  secondary  of  the  high  frequency  coil  will  depend  on  the 
width  of  the  '  'spark  gap, ' '  consequently,  in  exciting  a  tube  it  is  best  to  start 
with  the  '  *  spark  gap ' '  very  short,  then  gradually  increase  until  the  tube  is 
properly  excited.  When  the  terminals  of  the  secondary  high  frequency 
coil  are  separated  farther  than  five  inches,  a  spark  will  pass  from  the  sec- 
ondary to  the  primary  of  the  high  frequency  coil.  By  the  use  of  a  good 
insulating  oil  a  much  longer  spark  can  be  obtained  from  the  high  fre- 
quency coil,  but  for  exciting  Rontgen  ray  tubes  a  five-inch  spark  will  be 
sufficient. 


CHAPTER  XXX. 


CONDENSER  FOR  EXTREMELY  HIGH  POTENTIALS. 


A  condenser  for  high  potentials  that  is  commercial  has  been  a  prob- 
lem that  has  long  defied  complete  solution,  and  the  demand  for  one  that 
will  withstand  the  enormous  potentials  of  so-called  Tesla  currents  has 
been  only  partially  met  by  the  clumsy  and  ineffective  Leyden  jar.  The 
writer's  practical  experience  with  the  condenser  herein  described  bears 
him  out  in  offering  it  as  a  complete  solution  for  Tesla  currents  as  usually 
employed,  and  a  partial  solution  for  the  problem  cf  how  to  get  a  con- 
denser for  high  voltage  commercial  currents. 

This  condenser  has  a  capacity  of  about  .02  microfarad  according  to 
the  specific  inductive  capacity  and  thickness  of  the  glass  that  is  used.  It 
will  replace  a  battery  of  fifty  or  sixty  quart  Leyden  jars  and  will  only  oc- 
cupy the  space  of  a  couple  of  them.  If  it  is  stacked  in  banks  of  fifty  or 
sixty,  a  capacity  of  one  microfarad  could  be  obtained,  which  is  sufficient 
for  experimentation  on  commercial  circuits.  This  condenser  can  be 
made  by  the  veriest  amateur,  at  an  expense  not  exceeding  $2.50. 

Procure  of  some  good-natured  photographer  a  supply  of  old  nega- 
tives five  by  seven  inches  in  lateral  dimensions.  About  one  hundred  will 
be  needed.  Soak  them  in  hot  water  till  the  gelatine  film  has  dissolved, 
rinse  them  off,  and  when  dry  and  clean  they  are  ready  for  use. 

A  dealer  in  photographic  supplies  will  sell  ferrotype  plates  14  ins.  X 
10  ins,  for  not  more  than  four  cents  each.  As  each  plate  of  this  size  will 
make  four  condenser  plates,  the  total  cost  of  the  latter  will  not  exceed 
$i.  The  plates  should  be  laid  out  and  cut  as  shown  in  Fig.  262.  Through 
the  center  of  each  lug  should  be  drilled  a  J/6-in.  hole.  Procure  an  Edi- 
son-Lalande  jar  5  ins.  X  8  ins.  in  horizontal  sectional  dimension,  this  be- 
ing a  standard  size>  Select  the  jar  with  some  care,  being  sure  that  the 
bottom  and  sides  are  perfectly  flat,  for  otherwise  the  condenser  plates 
will  not  pack  in  place  nicely.  Two  pieces  of  ^J-in.  brass  rod  should  now 
be  obtained,  together  with  a  box  of  ^-in.  copper  rivet  burrs,  and  some 
'j6-in.  standard  tap  brass  nuts.  The  brass  rods  should  be  the  length  of 


CONDENSER  FOR  EXTREMELY  HIGH  POTENTIALS 


235 


the  Edison-Lalanclc  cell,  and  should  be  threaded  for  some  distance  on 
each  end.  Having  obtained  about  one-half  gallon  of  parafnne  oil,  the  con- 
denser is  ready  to  be  put  together. 

The  first  thing  to  do  is  to  pack  the  jar  full  of  glass  and  ferrotype 
plates,  so  adjusting  their  number  that  there  will  be  one  less  ferrotype 
than  glass  plate.  If  the  glass  is  not  too  thick,  the  jar  will  hold  between 
ninety  and  one  hundred  plates,  and  it  should  have  just  enough  that  the 


FIG.  262. — GLASS  PLATES. 


FIG*  263. — ARRANGEMENT  OF  PLATES. 


FIG.  264. TERMINALS  OF  PLATES. 


FIG.  265. — CONDENSER  COMPLETED. 


walls  of  the  jar  shall  be  effective  in  holding  the  plates  together  in  a  solid 
homogeneous  mass. 

The  plates  of  glass  and  sheet  iron  should  now  be  arranged  alternately, 
as  shown  in  Fig.  263.  The  lugs  of  each  set  of  plates  are  to  be  threaded 
with  the  brass  rods  before  mentioned,  and  rivet  burrs  interspersed  so  that 
when  the  nuts  are  set  up  as  shown  in  the  sketch  of  the  complete  con- 
denser (Fig.  265)  the  tin  plates  will  not  bind  the  glass  plates  between 
them.  The  terminals  may  be  simple  wires,  but  preferably  a  ball  and 


236  ELECTRICAL  DESIGNS 

knob  arrangement  as  shown  in  detail  in  Fig.  264  and  in  position  on  the 
condenser  in  Fig.  265.  After  the  condenser  is  thus  arranged,  it  remains 
to  fill  it  up  over  the  tops  of  the  lugs  with  paraffine  oil  and  it  is  complete. 

As  described,  the  condenser  would  be  suitable  for  potentials  of  io>- 
ooo  volts  or  less.  For  higher  potentials  the  plates  between  the  conduc- 
tors may  be  made  thicker.  This  will  reduce  the  capacity  of  the  con- 
denser both  by  increasing  the  thickness  of  the  dielectric  and  reducing  the 
number  of  plates  that  can  be  placed  inside  a  jar  of  given  dimensions. 

The  ball  and  knob  arrangement  is  very  simple.  Some  i-i6-in.  brass 
rod  is  bent  into  a  J^-in.  eyelet  at  one  end,  while  on  the  other  is  cast  al 
round  leaden  bullet.  These  rods  are  bolted  each  to  its  system  of  plates 
on  the  rod  holding  the  plates  together.  They  will  serve  to  separate  the 
ten  plates  at  the  points  where  they  are  bolted  in,  instead  of  washers,  and 
will  bind  a  sufficient  amount  to  hold  them  in  any  position  that  they  may 
be  placed ;  as  they  are  placed  opposite  each  other  the  discharge  gap  may 
be  varied  at  pleasure. 

For  use  with  the  higher  potentials  the  jar  had  better  be  of  hard  rub- 
ber, for  it  is  liable  to  be  punctured,  and  if  this  happens  the  jar  may  crack 
and  release  the  oil,  to  the  great  discomfiture  of  the  experimenter. 


CHAPTER  XXXI, 


CONSTRUCTION  OF  A  WIMSHURST  INFUJICNCE   MACHINE. 


This  machine  is  the  easiest  of  all  static  machines  to  make,  and  one 
of  the  most  satisfactory  in  its  results.  It  is  practically  independent  of  the 
weather  conditions.  If  made  as  described  herein,  the  machine  will  be 
capable  of  giving  a  continuous  stream  of  two-inch  sparks,  and  will  have 
sufficient  power  to  excite  a  small  Crookes  tube,  provided  that  the  termi- 
nals of  the  tube  are  very  near  together. 

The  first  and  most  difficult  part  of  the  work  is  to  shape  the  glass  discs. 
There  are  two  of  these  and  they  are  made  exactly  alike.  They  are  to  be 
twelve  inches  in  diameter,  and  have  a  "^4 -in.  hole  in  the  center.  Inas- 
much as  many  are  not  familiar  with  the  cutting  of  glass  into  such  a  shape 

a  few  hints  will  be  useful. 

Select  a  piece  of  window 
glass  of  the  cheap  green  variety. 
Better  grade  glass  contains  lead 
and  is  less  suitable.  The  hole  in 
the  center  should  be  bored  first. 
Prepare  a  solution  of  camphor 
in  turpentine  and  use  it  to  keep 
the  boring  tool  moist.  The  bor- 
ing tool  may  be  made  of  a  rat 
tail  file.  The  end  should  be  snap- 
ped off  and  the  boring  performed 

with  a  twisting  motion  of  the  hand,  care  being  taken  to  keep  the  file 
moist.  Patience  is  necessary,  and  when  the  hole  gets  so  deep  that  it  is 
nearly  ready  to  break  through,  it  is  necessary  to  proceed  with  extreme 
caution.  Once  safely  through,  the  hard  part  of  the  work  is  done.  The 
hole  must  now  be  cautiously  filed  to  size,  still  using  the  camphor  and  tur- 
pentine as  a  moistener.  A  mark  to  work  by  may  be  made  by  gluing  a 
piece  of  cardboard  carrying  a  hole  of  proper  size  onto  the  side  of  the 
glass. 


FIG.  266. — METHOD  OF  CUTTING  GLASS  DISCS. 


ELECTRICAL  DESIGNS 


:£fc  ±±1^4- 

FIG.  267. — HUBS  OF  MACHINE. 


FIG.   268. — STDE  SUPPORTS. 


FIG.    269. — SHAFT. 


SIDE  ELEVATION 


ELEVATION 

VIEW  AT  A   B 


Glass 


FIG.  270. — ELEVATION  OF  MACHINE. 


W1MSHURST  INFLUENCE  MACHINE 


239 


Having  completed  the  hole,  it  remains  to  trim  the  edge  of  the  glass 
into  circular  form.  This  is  a  comparatively  easy  matter.  Erect  on  a  flat 
surface  a  little  pillar  of  wood  24~m-  m  diameter.  Place  the  glass  over  this 
so  that  the  pillar  protrudes  through  the  hole.  Prepare  a  loop  of  string  of 
such  length  that  when  it  is  looped  around  the  pillar  as  in  Fig.  266,  the 
glazier's  diamond  will  swing  in  a  twelve-inch  circle.  Be  sure  to  use  a 
glazier's  diamond,  as  the  use  of  a  cheap  wheel  glass  cutter  would  be  likely 
to  spoil  all  the  work  in  boring  the  holes.  It  may  be  better  to  have  a! 
glazier  snap  off  the  glass  if  the  operator  is  not  experienced  in  such  work. 

Prepare  the  wooden  hubs  as  shown  in  the  drawing  (Fig.  267).  Bush 
them  with  a  brass  tube  2^-in.  in  internal  diameter.  These  hubs  are  se- 
cured to  the  glass  discs  with  cement.  Major's  cement  or  marine  glue 


FIG.  271. 

is  excellent,  and  bicycle  tire  cement  answers  very  well.  After  this  is  done 
the  discs  should  be  thoroughly  shellacked  with  filtered  shellac,  and  al- 
lowed to  dry.  In  the  meantime  other  parts  may  be  prepared. 

The  side  supports  are  of  wood,  and  hard  maple  is  preferable.  They 
are  finished  to  the  size  shown  in  the  drawing  (Fig.  268)  and  the  holes  in 
the  uppper  part  are  of  such  size  as  to  tightly  fit  the  ^-in.  shaft  they  sup- 
port. This  shaft  does  not  revolve.  The  hubs  with  their  glass  discs  re- 
volve upon  it. 

The  shaft  carrrying  the  two  wheels  is  the  only  part  that  requires  the 


240  ELECTRICAL  DESIGNS 

services  of  a  metal  lathe ;  should  this  not  be  available,  the  metal  parts  can 
be  made  for  a  small  sum  by  a  machinist  from  the  figured  drawings  in  this 
article.  In  its  largest  diameter  this  shaft  is  5^ -in.,  and  all  of  this  part  is 
threaded.  The  ends  are  turned  down  to  y2 -in.  journals,  as  shown  in  the 
drawing.  One  of  these  journals  is  sufficiently  long  to  pass  completeb 
through  its  bearing  and  carry  a  small  crank.  The  shaft  is  shown  in  Fig. 
269,  and  the  bearing  in  Fig.  270.  This  latter  may  be  cast  in  brass  from 
a  wooden  pattern. 

The  remainder  of  the  wooden  parts  of  the  machine  may  be  built  and 
assembled  as  per  drawing.  They  should  be  of  hard,  well-seasoned  ma- 
ple, and  thoroughly  varnished.  The  parts  should  be  put  together  with 
glue.  Nails  and  screws  are  to  be  avoided  They  will  be  necessary  to 
hold  the  main  supports  of  the  machine  in  place  and  in  some  other  places 
where  the  strain  is  great,  but  they  should  be  used  sparingly.  The  whole 
should  be  given  a  coat  of  shellac  varnish. 

-* 

IP                           ^  ©  ft  ) 

\J_L_ l  \  L/ 


us 


FIG.    272. — YOKE  FOR  CONNECTING  OPPOSITE  SECTIONS. 


FIG.    273. — COMB. 

When  the  discs  are  thoroughly  dry  they  are  ready  to  receive  the  tin- 
foil sectors  (Fig.  271).  There  are  twelve  of  these  to  each  disc,  and  they 
are  secured  in  place  at  equal  angular  intervals  thereon.  Follow  the  draw- 
ings closely  and  no  mistakes  can  be  made.  Shellac  is  to  be  used  as  an  ad- 
hesive, and  the  edges  of  the  sectors  are  to  be  covered  with  varnish,  over- 
lapping at  least  i-i6-in.,  to  prevent  dissipation  of  charge.  This  com- 
pletes the  discs. 

Mounted  on  the  'disc  shaft  with  a  tight  driving  fit  are  two  pieces  of 
hard  rubber  (Fig.  272).  These  carry  stiff  wires,  on  the  ends  of  which  are 


WIMSHURST  INFLUENCE  MACHINE 


241 


light  brushes  made  of  tinsel.  Each  rubber  piece  carries  two  brushes,  one 
at  each  end,  and  the  two  brushes  are  electrically  connected.  They  are 
adjusted  so  as  to  just  touch  the  sectors,  as  the  discs  rotate  and  thereby 
put  opposite  sectors  in  contact.  Their  angular  position  can  be  easily 
adjusted  to  the  position  where  the  working  of  the  machine  is  bound 
to  be  best. 

Two  U-shaped  combs  collect  the  output  from  the  discs.  They  are 
conveniently  made  by  drilling  a  X'm-  brass  rod  with  holes  at  suitable  in- 
tervals and  soldering  pin  points  into  the  holes.  The  combs  may  then 

be  bent  to  shape.  In  Fig.  273 
is  illustrated  themethodo?  form- 
ing the  comb.  The  sides  of  the 
enclosure  are  of  hard  rubber  and 
serve  to  support  the  combs.  A 
small  binding  post  may  be 
threaded  into  a  hole  at  the  curv- 
ature of  tlie  U  of  the  comb,  and 
with  the  aid  of  a  few  washers  the 
comb  is  neatly  and  securely  held. 
See  general  view,  Fig.  274. 

The  other  sides  of  the  en- 
closure are  of  glass,  both  on  ac- 
count of  its  insulating  quality 
and  transparency.  The  plates 
are  held  in  place  by  pieces  of 
rabbeted  moulding  mitered  on  to 
the  sides  of  the  upright  pillars. 
If  the  construction  is  followed 
out  as  shown  in  the  cuts,  the 
glass  and  rubber  plates  will  lift 
like  a  window  sash  and  render 
the  machine  completely  accessi- 
ble. The  discs  are  driven  in  op- 
posite directions  by  means  of  a 

straight  and  a  crossed  belt  from  the  shaft  below.  In  making  the  metal 
parts  of  the  machine,  all  sharp  corners  are  to  be  avoided  with  great  care, 
for  at  every  corner  the  charge  disappears  and  leaks  away. 

The  person  building  this  machine  must  not  be  disappointed  if  at  first 
trial  it  does  not  work  at  once.  If  the  shellac  is  the  least  particle  damp  the 
machine  will  refuse  to  generate,  but  once  dry  it  will  generate  without  fail- 
ure thereafter.  The  tinsel  brushes  must  make  positive  contact  with  the 


FIG.  274. — GENERAL  VIEW  OF  MACHINE. 


242  ELECTRICAL  DESIGNS 

sectors  or  the  machine  will  not  start.  They  must  be  so  adjusted  as  to 
touch  opposite  sectors  simultaneously.  The  best  working  angle  for  the 
tinsel  brushes  is  45°  with  the  horizontal.  The  discs  should  rotate  from 
the  comb  towards  the  nearest  tinsel  brush. 

The  entire  cost  of  the  machine,,  assuming-  that  all  of  the  metal  work- 
ing that  requires  the  use  of  machine  tools  is  hired  out,  should  not  ex- 
ceed $5. 


CHAPTER  XXXII. 


TELEPHONE  TRANSMITTER  AND   RECEIVER. 


The  only  thing  that  prevented  Philipp  Reis  being  honored  the  world 
Over  (as  he  is  to-day  in  Germany)  as  the  inventor  of  the  telephone,  was 
the  fact  that  he  could  not — or  those  who  have  since  tried  cannot — make 
his  first  instruments  talk.  It  is  said  that  the  difficulty  now  is  to  find  a  mi- 
crophonic  instrument  of  any  kind — his  kind  included — that  will  not  talk. 
And  all  the  reason  in  the  world  is  that  we  know  how  to  adjust  a  single 
screw!  The  whole  secret  lies  in  keeping  the  electrodes  together  con- 
stantly. This  is  the  only  real  difference  between  the  Reis  telephone  and 
the  Blake  transmitter,  which  is  in  use  all  over  the  world  and  lias  proved 
the  best  all-around  instrument  on  the  market. 

For  talking,  a  Blake  transmitter  and  a  form  of  the  standard  Bell  re- 
ceiver will  be  found  the  best.  The  patents  on  both  of  these  instruments 
have  expired,  and  they  can,  therefore,  be  made  and  used  by  anyone  at 
present. 

•  The  Blake  transmitter  is  illustrated  in  Figs.  275  to  281,  and  the  re- 
ceiver in  Fig.  282.  The  receiver  is  the  easier  to  construct  and  will  be 
described  first.  Procure  a  straight  bar  magnet  of  the  best  tool  steel, 
hardened  glass-hard  and  strongly  magnetized  (Tungsten  steel  is  prefer- 
able). It  should  be  long  in  proportion  to  its  thickness,  in  order  to  main- 
tain its  magnetism  well — say,  J4~m-  thick  by  7  ins.  long. 

Take  three-sheet  bristol  board  and  cut  out  two  discs,  about  il/2  ins. 
in  diameter.  Thin  hard  wood  discs  will  also  answer.  Cut  at  the  center 
of  each  a  hole  of  the  exact  size  of  your  magnet  and  slip  the  discs  on  one 
end  of  the  magnet,  ^4 -in.  apart.  One-sixteenth  of  an  inch  of  the  end  of 
the  magnet  should  protrude.  Wind  a  single  winding  of  thin  paper  around 
the  magnet,  between  the  discs,  for  insulation.  Then  wind  No.  36  silk- 
covered  copper  wire  carefully  between  the  discs  until  the  space  is  nearly 
full.  In  winding,  the  best  results  can  only  be  attained  by  the  greatest 
care.  There  must  be  absolutely  no  kinks  or  twists  in  the  wire  and  no 
breaks  in  the  insulation.  A  little  melted  paraffine  or  a  little  shellac  can 


244 


ELECTRICAL   DESIGNS 


be  placed  over  the  outside  layer  to  hold  the  wires,  and  you  should  leave 
several  inches  of  ends  for  connecting  up. 

Now  take  a  sound  piece  of  hard  wood  and  turn  up  a  case.  In  the  il- 
lustration (Fig.  282)  this  is  lettered  R.  It  should  be  I  in.  in  external 
diameter,  except  at  the  large  end,  which  should  spread  out  in  a  bulbous 
form,  as  shown,  with  a  diameter  of  2^2  ins.  outside  at  the  edge.  Turn  up 
a  cap  of  the  same  wood,  in  the  shape  shown.  Its  outside  diameter  is  3 
ins.,  and  it  should  fit  neatly  over  the  end  of  the  case.  The  cap  is  lettered 
A.  A  depression  should  be  turned  in  the  outer  face,  as  at  b,  and  a  sim- 
ilar, but  shallower,  depression,  as  shown  in  the  inner  face.  In  the  center 
should  be  a  hole,  j/2-in.  in  diameter.  Drill  small  screw  holes  in  the  side 
flanges  for  the  screws,  d. 

Through  the  center  of  your  case  bore  out  a  straight  smooth  hole  just 


FIGS.    275   AND   276. 

large  enongh  to  receive  the  magnet,  and  at  the  large  end  of  the  case 
turn  the  hole  out  to  2  ins.  in  width  and  ^-in.  deep,  as  shown. 

Drill  a  screw  hole  for  the  set-screw,  S,  near  the  small  end  of  the 
case.  Now  slip  the  long  end  of  the  magnet  into  place.  If  it  proves  too 
loose,  wrap  thin  paper  about  it. 

Take  a  pair  of  compasses  and  lay  off  on  cardboard  a  circle  with  i  }£ 
in.  radius — to  fit  exactly  between  the  cap  and  case.  With  this  as  a  tem- 
plate scratch  a  similar  circle  on  a  piece  of  photographer's  ferrotype  plate 
— "tintype"  plate — which  you  can  buy  for  a  nickel.  Cut  out  the  disc 
with  sharp  scissors.  Take  care  not  to  bend  it,  as  this  is  the  reason  the 
compasses  are  not  used  directly  on  it.  Any  bend  or  buckle  spoils  it. 
This  is  the  diaphragm.  Now  take  two  pieces  of  No.  16  wire,  scrape  the 


TELEPHONE  TRANSMITTER  AND  RECEIVER 


245 


ends  and  make  a  kink  in  each.  Solder  the  ends  of  your  fine  wire  coil  to 
them  and  pass  them  through  the  side  of  the  case.  They  are  shown  at  w 
in  Fig.  282.  The  object  of  the  kink  or  knot  becomes  apparent  when  you 
insert  it.  The  knot  comes  against  the  side  of  the  case,  and  prevents 
any  pull  coming  on  the  thin  wire  to  break  it.  Of  course,  if  you  want  to 
take  the  trouble  you  can  cut  a  channel  each  side  of  the  magnet  hole  all 
the  way  back  to  the  rear  cap,  c,  and  put  binding  posts  there.  Now  put 
the  diaphragm  half  over  the  large  end  of  the  case,  and  adjust  the  magnet 
rntil  it  nearly  touches;  i~32-in.  is  the  proper  clearance.  Then  screw 
down  the  set-screw,  S.  Put  on  the  diaphragm  (shown  at  c,  place  the 
cap,  a,  over  it,  screw  in  the  holding  screws,  d,  and  glue  on  a  covering 
disc,  e,  for  the  rear  end,  and  your  instrument  is  complete. 

It  is  advisable,  in  making  the  cap,  a,  to  have  the  inner  face  over  the 


Battery. 


Line 


FIG.   277- 

diaphragm  with  a  clearance  of  not  more  than  i-i6-in.  If  there  is  too 
much  space  between  the  cap  and  diaphragm  the  sounds  received  will  be 
muffled.  The  cap  should  be  firmly  adjusted  against  the  diaphragm  at  the 
edges  so  as  to  clamp  it  against  rattling. 

In  making  the  transmitter  the  first  thing  is  the  case,  A.  This  is 
shown  in  Figs.  275  and  276.  It  should  preferably  be  of  hard  wood,  nice- 
ly smoothed,  filled,  rubbed  and  polished.  The  door  of  the  case  carries 
all  the  operative  parts  of  the  transmitter  and  is  provided  with  brass  hinges, 
which  form  part  of  the  circuit,  so  that  by  simply  opening  the  door  yot: 
can  get  at  the  apparatus  without  breaking  any  connection.  The  mouth- 
piece, a,  is  simply  a  depression  turned  in  the  door  face,  as  shown  in  dot- 
ted lines  in  Fig.  276. 


246 


ELECTRICAL  DESIGNS 


The  transmitter  proper  is  all  carried  by  the  iron  ring,  B.  This  is 
best  shown,  with  its  dimensions,  in  Figs.  278  and  279.  As  seen,  it  is  a  flat 
ring  having  a  circular  rabbet  or  depression  to  receive  the  diaphragm, 
and  an  upper  and  a  lower  lug,  b,  and  c.  It  is  screwed  directly  on  the 
inside  of  the  door  by  screws,  as  shown  in  Fig.  277.  It  should  be  turned 
tip  smooth  and  true,  and  the  extreme  thickness  of  metal  may  be  about 
3/3  in.  on  the  edge.  The  upper  lug,  b}  is  tapped  in  its  face  for  two  screws, 
,  and  the  lower  lug,  c,  is  vertically  tapped  for  one,  the  adjusting  screw,  d. 
Carried  on  the  upper  lug,  by  the  spring,  c,  as  shown  in  Figs.  277  and  281, 
is  the  bar  D,  to  which  the  electrodes  are  attached.  This  bar  has  an 
tipper  or  head  lug,  /,  and  an  inclined  foot,  g.  The  head  lug  carries  the 
springs,  h  and  /,  carrying  the  electrodes,  E  and  p.  These  springs  are 
simply  clamped  to  the  head  lug,  /,  the  first  one,  h,  resting  directly 


FIGS.    278,   279,    280  AND  28l. 

against  the  metal,  and  held  on  by  the  insulating  block,  i,  against  which 
rests  the  spring,  /,  clamped  in  turn  by  another  little  insulating  block. 
The  screws  pass  through  both  insulating  blocks  and  are  tapped  into 
metal  head  lug,  /.  To  the  spring,  /,  above  the  insulation,  is  soldered  a 
wire,  forming  one  terminal  of  the  transmitter. 

In  operation,  the  point,  p,  carried  by  spring,  /,  rests  against  the  dia- 
phragm, the  spring,  e,  however,  constantly  tending  to  carry  it  away ;  so 
that  by  screwing  up  or  down  the  screw,  d,  the  pressure  may  be  accurately 
adjusted. 

The  spring,  h,  is  of  fine  spring  steel,  i-ioo  in.  thick  and  9-64  in. 
wide.  Secured  to  the  end,  either  by  clamping  under  a  screw  head,  or  by 
cutting  a  channel  across  and  upsetting  the  edges  over  the  spring,  is  a 


TELEPHONE  TRANSMITTER  AND  RECEIVER 


247 


brass  button,  E,  carrying  the  carbon  electrode,  k.  The  way  to  make  this 
is  as  follows : 

From  a  solid  piece  of  brass  turn  up  the  button  of  the  size  indicated, 
leaving  the  edges  very  thin  and  sharp.  Cut  the  carbon  button,  k,  accu- 
rately and  channel  around  it  a  shallow  groove  just  where  the  edge  comes. 
Then  put  the  whole  in  the  lathe  and  spin  the  edge  around  into  the 
channel  so  that  it  tightly  embraces  the  carbon. 

The  carbon  button,  k,  must  be  pure  homogeneous  carbon  free  from 
grit,  and  highly  polished  on  the  contact  surface.  The  way  to  get  the 


[FIGS.  282,  283  AND  284. 

best  polish  is  to  rub  the  button  for  a  while  on  a  smooth  sheet  of  the 
same  kind  of  carbon.  As  this  is  not  usually  available.,  however,  you  will 
probably  find  most  convenient  the  old  reliable  emery  paper  or  cloth. 
Take  a  piece  of  fine  emery  cloth  about  6  ins.  square  and  rub  your  button 
(which  you  must  leave  about  y%  in.  too  high  when  mounting)  on  the 
emery,  in  a  3  in.  circle.  Keep  it  moving  always  in  the  same  direction 
and  after  a  while  the  carbon  deposited  on  the  emery  will  form  a  fine 
polishing  surface,  and  give  you  a  glass  polish.  Be  sure,  however,  to  use 
none  but  the  finest  emery. 

Attached  to  the  spring,  /  (which  is  of  German  silver,  .005  in.  thick, 
y%  in.  wide),  is  the  platinum  point,  p.  This  can  best  be  secured  by  solder, 
and  should  be  5-64  in.  across.  A  tiny  end  of  platinum  wire  put  through 


248  ELECTRICAL  DESIGNS 

a  corresponding  hole  in  the  end  of  spring,  /,  and  soldered,  is  all  *hat  is 
required. 

In  the  bar,  D,  is  an  opening,  opposite  the  electrode  to  permit  adjust- 
ing. The  diaphragm,  C,  rests  in  the  depression  in  the  ring,  B.  Around 
its  periphery  is  stretched  a  rubber  band,  o,  to  deaden  or  dampen  the 
vibrations  to  some  extent.  It  is  held  in  place  by  two  spring  arms,  in  and 
«,  made  of  flat  spring  steel,  and  shown  best  in  Fig.  281.  The  arm,  n, 
extends  over  beyond  the  rubber  sleeve,  covering  the  end  that  rests  on  the 
diaphragm.  This  produces  a  dampening  effect  that  is  very  necessary 
because  of  the  delicacy  of  the  contacts  in  this  form  of  instrument.  The 
other  arm,  m,  simply  extends  on  to  the  rubber,  and  serves  merely  as  a 
clamp.  Both  spring  arms  should  press  lightly  on  the  diaphragm. 

The  diaphragm  itself  is  to  be  made  of  sheet  iron.  Ferrotype  iron, 
much  heavier  than  that  used  for  the  receiver,  is  required. 

One  side  of  the  circuit  through  the  transmitter  leads  from  the  iron 
ring,  B  (to  which  the  wire  is  soldered),  to  a  spring  on  the  lower  hinge,  H. 
This  spring  (one  on  each  hinge)  makes  a  scraping  contact  with  the  other 
leaf  of  the  hinge  when  the  door  is  shut,  and  so  ensures  a  good  contact 
there.  The  other  side  of  the  circuit  leads  from  the  spring,  /,  to  the  upper 
hinge.  The  current  from  the  battery  enters  at  binding  post  i,  Fig.  277, 
flows  to  the  upper  hinge,  through  the  wire  to  the  spring,  /,  platinum  tip, 
p,  carbon,  k,  brass  button,  E,  steel  spring,  h,  iron  bar,  D,  screw,  d,  lug,  c, 
ring,  B,  and  wire,  to  the  lower  hinge ;  thence  to  the  primary  of  the  induc- 
tion coil,  7,  to  the  second  binding  post  2,  and  so  back  to  battery. 

The  secondary  of  the  induction  coil  is  connected  to  the  binding  posts 
3  and  4.  The  induction  coil  itself  may  be  made  as  follows : 

Take  a  bundle  of  very  soft  and  fine  iron  wires,  2j4  ins.  long,  and 
enough  to  measure  *4  m-  or  H  m-  through.  Wrap  a  turn  or  two  o(  thin 
tough  paper  about  them,  and  fit  on  either  end  a  square  block  of  wood, 
Y%  in.  thick  and  iy&  ins.  on  a  side.  Wind  between  these  blocks  and  on 
the  paper,  about  35  ft.  of  No.  24  silk-covered  wire.  The  ends  should 
be  carried  out  through  fine  holes  drilled  in  the  wooden  end  pieces. 

Over  this  primary  winding  lay  on  carefully  about  600  ft.  of  No.  3?, 
fine  silk-covered  wire,  and  carry  the  ends  out  at  one  end  in  a  similar 
manner.  Cover  the  coil  with  a  wrapping  of  binder's  paper  gummed 
fast  on  the  edges,  and  fasten  the  coil  in  the  position  shown  in  Fig.  277, 
by  two  long  screws  through  and  through  the  ends  into  the  back  of  the 
case.  Carry  the  ends  of  the  secondary  winding  to  binding  post  screws,  3 
and  4,  and  solder  them.  Connect  one  end  of  the  primary  to  binding 
post  i  and  the  other  to  the  lower  hinge,  as  shown. 


TELEPHONE  TRANSMITTER  AND  RECEIVER  249 

In  winding  it  is  important  to  wind  in  regular  layers  from  end  to  end, 
and  to  avoid  the  slightest  kink  or  twist  in  the  wire. 

The  connections  of  the  instrument  are  clearly  shown  in  Fig.  284. 
The  switches  cut  off  the  bell  and  put  on  battery,  when,  moved  to  the 
right,  for  calling,  and  cut  in  the  telephone  and  close  the  local  battery  for 
talking  when  moved  to  the  left. 

If  an  outdoor  line  is  used  it  is  advisable  to  use  some  form  of  light- 
ning arrester,  which  may  be  obtained  from  a  dealer  at  small  cost,  outside 
the  instrument. 

In  fitting  up  a  telephone  line  for  communication  there  are  four  ele- 
ments necessary  at  each  end — a  transmitter,  a  receiver,  a  call-sending 
device,  and  a  call-receiving  device.  For  the  purposes  of  this  article  I  will 
presume  that  the  telephone  line  is  a  short  one,  say  less  than  a  mile  in 
length — perhaps  1000  yds.,  with  a  single  wire,  of  iron,  No.  12,  galvanized. 
At  each  station  you  bring  the  line  indoors  to  the  instrument  by  connect- 
ing office  wire  at  the  window  and  leading  it  around  the  woodwork  of  the 
room.  The  joint  outside  the  window  must  be  soldered,  the  joint  taped, 
and  the  wire  bent  down  U-shaped  before  it  comes  in,  to  allow  moisture 
to  drip  off.  At  your  instrument  another  piece  of  office  wire  should  be 
started  and  led  off  to  the  nearest  water  pipe.  The  end  of  the  wire  should 
be  stripped  for  12  ins.,  cleaned  bright,  the  pipe  likewise  scraped  bright, 
and  the  wire  wound  tightly  around  the  pipe  and  soldered.  If  this  is  done 
carefully  at  both  ends  of  the  line,  you  have  a  good  circuit  completed 
over  the  iron  wire  from  one  station  to  the  other  and  back  by  way  of  the 
pipes  and  the  earth.  It  only  remains  to  connect  your  instruments  to  the 
wire  ends  and  you  should  be  able  to  talk  perfectly. 

For  such  a  line  a  push  button  and  vibrator  bell,  with  a  battery,  at 
each  end,  will  furnish  as  good  a  call  as  may  be.  The  arrangement  of 
these  is  indicated  in  Fig.  284.  They  can  be  purchased  of  any  supply 
dealer  more  cheaply  than  you  can  make  them. 


CHAPTER  XXXIII. 


CONSTRUCTION  OF  A   DRY' 'BATTERY 


Dry  batteries,  so  called,  are  only  dry  in  the  sense  that  there  is  no 
fiuid  spilled1  or  slopped  over  when  they  are  shaken  or  overturned.  In 
every  voltaic  cell  the  current  is  derived  from  the  chemical  action  which 
goes  on  within  its  substance,  and  no  chemical  action  can  take  place  be- 
tween solids  alone,  but  in  all  cases  there  must  be  present  a  liquid  or  a  gas. 
Some  few  cells  employ  gaseous  electrolytes,  and  some,  fused  salts,  but 
the  vast  majority  use  aqueous  solutions  of  their  respective  chemicals,  and 
it  is  in  this  class  that  the  ordinary  dry  cells  are  to  be  found.  Even  the 
old  dry  piles  of  Zamboni  and  others,  which  consisted  of  discs  of  paper 
coated  with  metals  (gold  and  silver  paper)  laid  up  "dry,"  in  reality  con- 
tained a  very  small  amount  of  moisture  in  the  paper,  and  if  the  paper  is 
really  perfectly  dry,  the  piles  will  not  work.  If  the  ordinary  dry  cell  then 
requires  moisture  to  make  it  work,  and  is  in  fact  only  a  non-spilling  wet 
cell,  it  is  a  natural  inference  that  the  wetter  the  cell,  consistent  with  it  not 
spilling  or  slopping  over,  the  better.  This  inference  is  absolutely  cor- 
rect. The  more  fluid  a  dry  cell  contains  the  better,  for  many  wet  cells 
would  be  improved  for  having  a  larger  amount  of  electrolyte  than  they 
do  have. 

It  might  seem  in  view  of  what  has  been  said,  that  any  wet  cell,  if  well 
sealed  up,  would  do  for  a  dry  cell,  but  such  is  not  the  case,  for  several 
reasons.  Many  cells  will  not  stand  sealing  up  tight,  because  they  give 
off  gases,  and  these  must  have  free  vent,  and  again  it  is  not  always  prac  • 
ticable  to  seal  up  a  cell  so  that  it  will  not  leak  at  all  when  inverted.  Again, 
it  is  not  worth  while  to  seal  up  a  cell,  except  one  of  the  kind  that  will 
last  for  considerable  time  before  it  gives  out  or  even  needs  replenishing. 
Another  desideratum  of  a  dry  cell  is  that  it  should  not  be  easily  broken, 
as  there  are  many  places  where  cells  are  liable  to  fracture  as  well  as  up- 
setting, etc.  For  these  reasons,  it  is  customary  to  dispense  with  the 
glass  jar,  and  to  make  the  zinc  serve  the  double  purpose  of  containing 
jar  and  electrode,  and  further,  to  use  an  absorbent  substance  that  wil? 


CONSTRUCTION  OF  A  DRY  BATTERY  CELL  251 

take  up  and  hold  the  fluid  electrolyte  like  a  sponge,  so  that  the  seal  is 
rather  to  prevent  evaporation  and  creeping-  cf  salts,  than  spilling  or  slop- 
ping. The  types  of  cells  giving  the  best  results  on  open  circuit  work, 
as  wet  cells,  naturally  do  the  best  when  put  up  in  the  dry  form ;  conse- 
quently, as  might  be  expected,  the  vast  majority  of  dry  cells  on  the 
market  are  some  form  or  variety  of  the  sal-ammoniac  type.  Several  of 
the  manufacturers  of  dry  cells  claim  to  have  valuable  secrets  relating  to 
their  manufacture,  but  however  true  this  may  be  in  regard  to  the  details, 
the  main  requirements  are  well  understood. 

In  making  a  dry  cell,  the  first  thing  requiring  attention  is  the  jar. 
This,  as  before  remarked,  is  usually  made  of  zinc.  The  cell  is  usually 
cylindrical,  although  sometimes  square  or  oblong  in  section,  but  in  any 
case  a  piece  of  moderately  heavy  sheet  zinc  is  bent  into  the  required 
form,  the  edges  soldered  together  and  a  bottom  soldered  in.  Any  one 
who  tries  to  solder  zinc  for  the  first  time  may  be  very  much  surprised 
and  disgusted  to  find  that  it  does  not  take  kindly  to  soldering  like  tin 
plate,  but  balks  and  makes  lots  of  trouble.  However,  by  observing  the 
proper  precautions,  zinc  may  be  soldered  with  comparative  facility. 
Thoroughly  clean  all  the  parts  where  it  is  intended  that  the  solder 
should  stick,  by  scraping ;  use  clean  chloride  of  zinc  for  a  flux,  and  apply 
the  solder  as  near  the  point  where  it  is  needed  as  possible,  not  trying  to 
make  it  flow  over  the  surface  of  the  zinc  as  can  be  so  readily  done  with 
tin  plate,  for  the  more  the  solder  alloys  with  the  zinc,  the  more  intract- 
able it  becomes.  Learn  to  make  a  joint  quickly  on  the  first  application 
of  the  soldering  bit,  as  the  more  you  fuss  and  tinker  with  it  the  rougher, 
more  unsightly  and  more  uncertain  it  becomes.  Another  pleasant  little 
habit  of  zinc  is  to  strip  the  tinning  off  the  soldering  bit.  You  may  have 
tinned  your  bit  with  the  utmost  care,  but  after  using  it  a  short  time  find 
it  completely  stripped.  Some  persons  prefer  an  iron  bit  to  the  usual 
copper  one,  claiming  that  it  holds  the  tinning  better,  although  somewhat 
more  difficult  to  tin  in  the  first  place. 

Having  made  the  jar,  the  next  thing  to  attend  to  is  the  contents.-, 
One  of  the  most  important  constituents  of  the  contents  is  the  absorbent. 
Several  materials  have  been  used  for  this  purpose,  among  which  are 
plaster  cf  paris,  gelatinous  silica,  gelatine,  so  called,  which  is  really  the 
starchy  mass  obtained  from  boiling  Irish  moss ;  gelatinous  magnesium 
oxychloride  and  a  material  made  from  the  granular  portion  of  the  rind 
of  the  cocoanut,  called  cofferdam.  There  are  other  materials,  but  these 
r.re  the  more  important  ones.  As  the  process  of  filling  the  cell  differs 
somewhat  for  each  kind  of  filling,  it  is  better  to  describe  each  one  sep- 


252  ELECTRICAL  DESIGNS 

arately.  The  zinc  usually  has  a  brass  binding  post  soldered  to  its  rim 
on  one  side  (there  being  no  objection  to  this  structure  in  a  dry  cell, 
because  the  electrolyte  cannot  possibly  come  in  contact  with  the  junc- 
tion), and  the  other  electrode,  which  is  usually  of  carbon,  has  a  binding- 
post  of  the  same  kind,  fastened  in  one  of  several  different  ways. 

The  seal  is  made  of  pitch  or  some  similar  material,  which  will  form 
an  air  and  water  tight  stopper,  and  also  resist  the  tendency  of  the  salts 
to  creep,  as  does  the  paraffin  coating  on  the  upper  part  of  the  jar  of  an 
ordinary  sal-ammoniac  cell.  It  is  simply  melted,  poured  in  on  top  of  the 
charge  and  allowed  to  cool  in  most  dry  cells,  but  some  manufacturers 
make  a  sort  of  safety  valve  or  pressure  regulator,  by  inserting  a  small 
piece  of  rattan  so  that  it  passes  completely  through  the  seal,  its  ends  pro- 
jecting slightly  above  and  below  the  pitch.  The  natural  porosity  of  the 
rattan  is  sufficient  to  relieve  any  pressure  generated  by  the  escape  of 
gases,  but  it  will  not  of  course  provide  for  the  swelling  of  the  more 
solid  portion  of  the  contents  which  sometimes  takes  place  when  the  ceil 
is  subjected  to  too  high  a  temperature,  and  cells  are  frequently  destroyed 
by  bursting  when  placed  in  boiler  rooms  and  other  situations  where  the 
temperature  rises  to  an  inordinate  degree. 

One  of  the  important  qualities  of  a  dry  cell  is  long  life  on  open  cir- 
cuit, which  means,  that  the  local  action  should  be  negligible,  and  it  is  in 
the  prevention  of  local  action  that  some  manufacturers  claim  to  have 
valuable  secrets.  Bi-sulphate  of  mercury  is  sometimes  used  10  keep  the 
zinc  amalgamated,  as  in  wet  cells.  This,  of  course,  does  some  good,  but 
it  is  not  all.  Anything  that  will  tend  to  keep  the  chemical  composition 
of  the  electrolyte  uniform  in  all  parts  of  the  cell  will  help  to  prevent 
local  action. 

We  will  now  consider  some  of  the  particular  forms  of  dry  cells.  The 
Cox  cell  is  formed  by  boiling  Irish  moss  in  sal-ammoniac  solution  until 
it  is  thoroughly  gelatinized,  and  pouring  it  into  the  zinc  jar,  where  the 
carbon  electrode  has  been  already  placed.  Bin-oxide  of  manganese  in 
conjunction  with  the  carbon  as  a  depolarizer  is,  of  course,  used  as  in  the 
wet  form.  The  inventor  also  mixes  a  little  bi-sulphate  of  mercury  with 
the  electrolyte.  Some  of  the  other  sal-ammoniac  cells  as  described  do 
not  use  it,  but  there  is  no  reason  why  they  should  not,  and  the  reader 
should  understand  that  he  may  use  it  or  not  in  the  other  cells  described. 
When  the  moss  solution  is  cold  it  sets  to  a  firm  jelly,  and  is  then  ready 
to  be  sealed.  Obach's  cell  is  made  by  mixing  plaster  of  paris  with  tlie 
sal-ammoniac  solution  and  pouring  it  into  the  jar  to  harden.  Mehner's 
cell  is  made  by  mixing  the  sal-ammoniac  solution  with  chloride  of  c^?- 


CONSTRUCTION  OF  A  DRY  BATTERY  CELL  53 

/ 

cium  and  calcined  magnesia,  forming  a  paste  of  about  the  consistency 
of  cream,  which  is  poured  into  the  jar,  and  in  two  or  three  days  forms  a 
stiff  jelly,  owing  to  the  formation  of  oxy-chloride  of  magnesium.  In 
Gassner's  cell  the  following  composition  is  used :  Oxide  of  zinc,  I  part ; 
sal-ammoniac,  I  part ;  plaster  of  paris,  3  parts ;  chloride  of  zinc,  i  part, 
and  water,  2  parts,  all  by  weight.  The  oxide  of  zinc  is  intended  to  make 
the  plaster  more  porous,  giving  this  cell  an  advantage  over  the  simple 
plaster  cell  before  described. 

Gelatinous  silica  is  precipitated  when  any  strong  acid  is  added  to  a 
solution  of.  silicate  of  soda,  and  several  inventors  have  used  silicate  of 
soda  to  gelatinize  the  electrolyte  in  storage  cells.  This,  of  course,  intro- 
duces sulphate  of  soda  into  the  electrolyte,  but  this  does  no  harm,  and  is 
even  regarded  as  beneficial  by  some.  The  charge  and  discharge  rate  of 
the  cell  is  much  reduced,  and  it  will  not  do  to  allow  much  gas  to  be 
generated  into  the  jelly,  neither  can  the  cell  be  sealed  up  perfectly  tight, 
but  a  vent  must  be  left  for  the  escape  of  gas.  The  spattering  or  spraying 
during  charge,  however,  is  cured,  even  if  no  cover  is  used. 

Gelatinous  silica  may  be  used  in  any  electrolyte  in  which  the  sodium 
salt,  formed  at  the  same  time  with  the  silica,  is  not  detrimental.  It  is  very 
difficult  to  wash  the  silica,  and  it  does  not  pay  to  do  it  for  such  a  pur- 
pose as  this,  so  if  it  is  intended  to  use  the  silica  with  sal-ammoniac  it  is 
better  to  precipitate  it  with  hydrochloric  acid,  instead  of  sulphuric,  as  in 
that  case  the  precipitated  silica  will  contain  chloride  of  sodium,  instead 
of  sulphate. 

The  Germain  cell,  which  at  one  time  attracted  considerable  atten- 
tion, used  the  material  known  as  cofferdam,  previously  mentioned  in  this 
article.  The  containing  vessel  of  this  cell  is  made  of  wood  boiled  in 
paraffine,  the  carbon  plate  is  imbedded  in  lumps  of  peroxide  of  manga- 
nese and  carbon,  and  the  rest  of  the  space  is  filled  with  cofferdam  satu- 
rated with  sal-ammoniac,  the  zinc,  which  is  well  amalgamated,  is  laid  on 
top,  and  the  cover  (or  rather  the  side)  is  screwed  on,  slightly  compressing 
the  contents. 

It  will  be  observed  that  some  of  the  directions  for  the  manufacture 
of  dry  cells  are  very  particular  about  the  amalgamation  of  the  zinc, 
even  when  the  electrolyte  is  sal-ammoniac,  but,  as  a  matter  of  fact,  most 
of  the  dry  cells  on  the  market  are  made  with  unamalgamated  zincs,  which 
is  a  pretty  good  proof  that  the  amalgamation  is  superfluous.  There  have 
been  many  other  dry  cells  proposed,  some  quite  elaborate  in  composi- 
tion and  others  less  so,  but  the  different  forms  of  the  sal-ammoniac  cell 
are  at  the  present  time  used  to  the  exclusion  of  everything  else. 


254  ELECTRICAL  DESIGNS 

One  practical  difference  between  wet  and  dry  cells  is  the  different 
manner  of  using-  them.  In  the  wet  cell,  we  endeavor  to  use  some  of  the 
parts,  such  as  the  containing  jar,  indefinitely,  and  the  porous  jar,  if  there 
is  one,  at  least  a  very  long  time.  We  also  endeavor  to  use  up  the  zinc  as 
completely  as  possible,  excepting  to  renew  the  electrolyte  several  times 
before  the  zinc  is  all  gone.  With  dry  cells,  however,  it  is  tbe  common 
practice  to  throw  them  away  as  soon  as  they  fail  from  any  cause  to  do 
their  work.  The  zinc  being  also  the  containing  jar,  it  manifestly  cannot 
be  all  used  up,  but  on  the  other  hand  must  be  discarded  as  soon  as  it  is 
perforated,  if  indeed  something  else  does  not  give  out  before  this  event, 
as  is  intended. 

Dry  cells  are  intended  more  particularly  for  a  class  of  consumers 
•who  do  not  care  to  be  bothered  with  the  manipulation  of  wet  cells.  When 
they  come  from  the  factory  they  are  ready  for  use  without  any  prepara- 
tion whatever,  and  when  they  are  exhausted  they  may  be  thrown  away 
Vvrithout  compunction,  for  they  are  made  for  such  a  low  price  that  it  does 
not  pay  to  spend  much  time  or  trouble  on  them,  even  when  they  happen 
to  be  in  such  a  condition  that  they  may  be  restored,  which  is  not  usually 
the  case. 

The  zinc  electrode-jar  must  be  insulated  in  some  way,  especially 
when  several  of  the  cells  are  used  in  series,,  as  in  this  case  any  external 
conductor  touching  two  of  the  cells  would  short  circuit  at  least  one  of 
them.  For  this  reason  they  are  usually  varnished,  and  often  in  addition 
placed  in  strawboard  boxes. 

Judging  from  the  variations  in  the  different  cells  on  the  market,  and 
which  work  satisfactorily,  it  would  appear  that  the  exact  proportions  of 
the  ingredients  are  not  very  important.  In  the  nature  of  things  the  elec- 
trolyte is  the  first  thing  to  give  out,  and  there  is  no  danger  of  getting 
too  much  of  it,  and  the  solution  should  be  saturated.  The  carbon  and 
manganese  should  not  take  up  too  much  space,  and  as  in  the  wet  sal- 
ammoniac  cells  the  manganese  is  usually  thrown  away  before  it  is  ex- 
hausted, from  the  fact  that  the  inner  part  of  each  lump  is  unavailable,  it 
is  evident  that  a  small  amount  of  manganese  Vv'ill  answer  the  purpose,  if 
arranged  so  as  to  be  available,  for  which  purpose  it  is  better  to  have  it 
rather  finely  broken  than  in  large  lumps,  and  in  as  intimate  contact  as 
possible  with  the  carbon.  There  being  no  danger  in  a  dry  cell  that  the 
manganese  will  become  displaced  after  having  been  once  fixed,  there  is 
no  objection  to  its  being  i:i  a  state  of  powder. 


CHAPTER  XXXIV. 


SOME   HANDY   COMMUTATOR   TOOI.S. 


Direct  current  dynamos  and  motors  have  now  come  into  very  gen- 
eral use,  exceeding  in  number,  perhaps,  steam,  gas  and  oil  engines  com- 
bined. As  is  well  known  these  electrical  machines  are  subject  to  mechan- 
ical wear  £t  only  the  bearings  and  the  commutator,  which  have  to  be 
replaced  from  time  to  time.  The  very  general  demand  for  dynamo  and 
motor  repairs  has  been  met  to  some  extent  by  electric  repair  shops  that 
have  come  into  existence  at  many  points  through  the  country,  but  aside 
from  these  there  is  hardly  a  regular  machine  shop  of  any  size  that  is  not 
constantly  called  on  for  more  or  less  work  on  the  worn  parts  of  dynamos 
and  motors. 

As  bearings  in  electrical  machines  are  usually  fitted  with  bronze 
bushings,  with  standard  reamed  holes,  well  equipped  machine  shops  are 
usually  in  position  to  make  these  parts,  but  in  the  matter  of  commuta- 
tors there  is  not  one  regular  shop  in  fifty  with  the  simple  tools  necessary 
for  their  renewal,  and  the  average  machinist  has  but  slight  conception  of 
how  this  work  must  be  done  to  insure  satisfactory  results. 

In  electrical  repair  shops  there  are  usually  some  tools  for  handling" 
commutator  work,  but  they  are  frequently  of  the  crudest  kind  and  such 
as  to  require  too  much  labor,  and  even  then  lack  certainty  in  results. 
Some  even  among  dynamo  and  motor  manufacturers  lack  the  few  and 
simple  tools  necessary  to  insure  first-class  commutator  construction  with 
a  minimum  of  labor.  As  usually  constructed,  a  commutator  contains 
a  number  of  copper  "bars"  or  "segments,"  separated  from  each  other 
and  the  clamping  parts  by  mica  strips  and  rings.  These  bars  are  parts 
of  true  circular  sectors,  though  not  of  the  exact  circle  of  the  commutator 
surface,  and  are  held,  with  the  intervening  mica  strips,  by  a  sleeve  and 
clamps. 

Some  of  the  main  requirements  of  commutator  construction  are  that 
each  segment  be  insulated  or  free  from  metallic  contact  with  any  other 
or  the  sleeve  and  clamps,  that  each  segment  be  so  firmly  held  that  the 


ELECTRICAL  DESIGNS 


forces  of  expansion,  due  to  heat  when  the  machine  is  in  use,  shall  not  alter 
its  relation  to  the  other  segments,  and  further  that  the  relative  position  of 
segments  and  mica  strips  shall  remain  the  same  after  the  commutator  has 
cooled. 

Were  all  parts  of  commutators  metal,  the  above  requirements  would 
make  careful  work  necessary,  but  as  each  segment  must  be  held  entirely 
by  contact  with  mica,  the  problem  is  much  more  difficult,  in  fact,  it  has 


FIGS.   285  AND  286. 

required  more  study  and  experiment  than  any  other  mechanical  question 
that  electrical  manufacturers  have  had  to  meet. 

Mica  lias  come  into  very  general  use  for  commutator  insulation  be- 
cause of  its  high  insulating  properties,  non-injury  by  heat  and  power  to 
sustain  great  pressures  with  but  small  compression.  On  dynamos  and 
motors  of  moderate  capacity  in  common  use  the  number  of  commutator 
segments  varies  from  six  to  eight  to  about  two  hundred  according  to  the 
purpose  and  capacity  of  the  machine,  the  most  common  numbers  of  seg- 
ments being  from  twenty-four  to  one  hundred. 


SOME  HANDY  COMMUTATOR  TOOLS  257 

As  for  every  segment  there  must  be  a  strip  of  mica,  in  addition  to 
the  mica  rings,  the  number  of  separate  parts  that  must  be  held  in  their 
exact  position  in  a  commutator  will  vary  from  fifty  to  about  four  hun- 
dred in  machines  of  moderate  capacity  and  common  use.  In  order  to 
hold  so  many  pieces  of  materials,  to  a  large  extent  contrary  as  to  their 
qualities,  rigidly  together,  it  has  been  found  necessary  to  assemble  them 
with  great  pressure,  and  then  set  the  permanent  clamps  as  tightly  as  pos- 
sible before  the  external  pressure  is  removed. 

The  two  most  common  and  successful  methods  of  clamping  com- 
mutators are  shown  in  Figs.  285  and  286,  the  solid  black  lines  in  each 
case  representing  the  mica  strips  and  rings.  The  better  class  of  segments 
are  forged  or  of  drawn  stock,  so  that  no  labor  is  required  oil  the  sides 
before  assembly  in  the  way  shown.  As  soon  as  assembled  it  is  necessary 
to  compress  and  hold  the  segments  securely  so  that  the  surfaces  which 
come  in  contact  with  the  mica  rings  may  be  machined. 

A  method  to  compress  and  hold  the  segments,  common  in  many 
electric  repair  shops  and  with  some  manufacturers,  employs  a  solid 
forged  ring  turned  on  the  inside  to  the  diameter  which  the  segments  are 
estimated  to  have  when  compressed,  and  tapered  slightly  at  one  edge 
so  as  to  start  easily  over  the  segments.  In  the  correct  use  of  this 
ring  it  should  be  forced  over  the  segments  with  a  pressure  of 
some  tons,  as  this  source  of  pressure  is  the  only  one  to  bring 
the  segments  and  mica  strips  solidly  together.  To  do  a  good  job 
with  this  solid  ring  a  hydraulic  or  large  screw  press  is  necessary,  and 
in  many  cases  machine  and  repair  shops  are  without  either  of  these 
presses,  so  that  the  ring  can  only  be  forced  on  and  off  the  segments  with 
a  hammer,  a  very  unsatisfactory  method.  A  serious  objection  to  this 
solid  ring  method  is  that  it  is  very  hard  to  estimate  the  exact  diameter 
to  which  the  ring  should  be  turned  in  order  to  properly  compress  the 
segments,  and  the  trials  to  see  how  hard  the  ring  crowds  on  all  take 
time.  Again  a  solid  forged  ring  must  be  had  for  every  size  of  commuta- 
tor, even  though  they  vary  by  only  a  small  fraction  of  an  inch  in  diam- 
eter, and  there  is  great  temptation  when  a  ring  goes  on  too  easy  to  let  it 
go  as  "good  enough."  To  do  away  with  the  necessity  for  presses,  also 
forged  rings  for  every  commutator,  save  time  and  insure  means  for  the 
desired  compression  in  every  case,  the  tool  shown  in  Fig.  287  is  now 
much  used. 

This  tool  consists  of  two  rings,  the  outer  a  solid  forging  and  the 
inner  an  iron  casting,  split  along  one  side  so  that  its  diameter  may  be 
slightly  changed. 


258  ELECTRICAL   DESIGNS 

The  outer  forged  ring  is  fitted  with  six,  eight  or  more  radial  set 
screws,  which  bear  upon  the  inner  split  ring.  The  commutator  segments 
and  mica  strips  having  been  assembled  in  circular  form,  the  split  cast- 
iron  ring,  having  been  turned  as  near  as  possible  to  the  correct -diameter, 
is  pushed  over  them  and  pressure  applied  by  means  of  the  large  set 
screws  in  the  forged  ring.  Any  desired  pressure  can  be  obtained  by  the 
combined  action  of  the  heavy  set  screws,  the  split  ring  readily  conforming 
to  the  slightly  reduced  diameter  of  the  segments.  The  work  can  also  be 
done  much  more  quickly  than  when  a  solid  ring  and  press  are  used.  I: 
is  not  necessary  to  have  one  of  the  above  tools  for  each  size  of  commuta- 


FIG.  287. 

tor,  as  only  the  inner  cast  ring  need  be  changed  until  the  commutator 
diameters  vary  by  as  much  as  three  inches,  so  that  the  forged  ring  be- 
comes either  too  small  or  so  large  as  to  be  unhandy.  In  this  way  two  or 
three  forged  rings  will  cover  a  large  line  of  commutators,  while  the  cast 
ring  for  each  commutator  is  cheaply  and  quickly  made. 

When  the  segments  are  securely  clamped  in  the  ring  the  next  step 
is  to  turn  up  the  surfaces  to  which  the  mica  rings  transmit  the  pressure ; 
the  lathe  chucks  are  the  only  means  for  holding  the  clamped  commutator 
segments  while  the  surfaces  at  each  end  are  machined,  and  here  comes 
a  waste  of  time  and  inaccurate  results  due  to  the  effort,  after  one  end 
of  the  segments  has  been  turned  up  to  reverse  them  in  the  chuck,  so  as  to 
machine  the  other  end  in  line  with  the  first.  The  tendency  with  work 
done  in  this  way  is  to  force  the  permanent  clamps  slightly  out  of  line 
with  each  other,  and  this  may  result  ia  loose  segments  at  some  point  in 


SOME  HANDY  COMMUTATOR  TOOLS  259 

tne  commutator.  Proper  expansion  mandrels,  as  shown  in  Figs.  288  and 
289,  not  only  enable  the  finished  surfaces  at  the  two  ends  of  the  seg- 
ments to  be  brought  practically  into  line,  but  also  save  much  time  on  the 
work. 

Fig.  288  shows  a  mandrel  adapted  for  use  with  segments  of  the 
type  in  Fig.  285,  where  there  is  no  undercut  work  to  be  done,  while  the 
mandrel  of  Fig.  289  is  more  convenient  for  undercut  segments.  The 
mandrel  of  Fig.  288  mounts  in  the  usual  way  on  lathe  centers,  has  a  taper 
of  one  in  twenty-four,  is  fitted  with  cast-iron  expansion  sleeve  and  a  screw 
collar  at  each  end,  to  force  the  sleeve  on  and  off.  The  cast-iron  sleeve 
should  be  cut  entirely  through  once  along  its  entire  length  and  nearly 
through,  say,  to  within  one-fourth  or  three-eighths  inch  at  three  or  more 
other  points,  that  it  may  expand  as  evenly  as  possibly  when  forced  onto 
the  mandrel. 

A  sleeve  for  this  mandrel  should  be  turned  outside  to  correspond 
with  the  inside  diameter  of  the  commutator  segments  it  is  intended  to 
mount,  and  if  very  accurate  work  is  desired  the  inside  of  segments 
should  be  turned  out  before  mounting  them  on  the'  expansion  sleeve, 
though  some  makers  think  this  unnecessary.  When  the  segments  are 
mounted  on  the  sleeve  this  latter  is  expanded  by  forcing  it  on  the  man- 
drel with  the  screw  collar,  and  the  ends  of  segments  in  Fig.  285  can  be 
turned  up  without  changing  their  position. 

The  segments  shown  in  Fig.  286  can  also  be  turned  on  the  above 
mandrel,  but  the  work  on  the  undercut  is  done  at  a  disadvantage  and 
much  time  can  be  saved  by  the  use  of  the  mandrel  of  Fig.  289.  This 
mandrel,  like  the  other,  is  on  a  taper  and  fitted  with  expansion  sleeve, 
but  one  end  is  forged  into  a  flange,  adapted  to  bolt  to  a  face  plate  and 
allow  free  access  to  one  end  of  the  commutator  segments,  so  that  the 
undercut  can  be  made  quickly.  One  end  of  the  segments  being  fin- 
ished, they  are  forced  off  the  mandrel  with  the  sleeve  by  the  screw  collar, 
and  then  put  on  again  reversed  and  the  other  end  finished.  There  is, 
of  course,  no  reason  to  take  the  mandrel  from  the  face  plate  until  both 
ends  of  the  segments  are  finished,  so  that  time  is  saved  and  the  finished 
surfaces  brought  very  nearly  in  line.  Quite  a  number  of  expansion 
sleeves  can  be  used  on  the  same  mandrel  for  different  commutators,  so 
that  two  or  three  mandrels  will  be  enough  for  a  factory  turning  out  a 
fair  line  of  machines. 

Having  turned  up  the  ends  of  the  segments,  it  is  next  necessary  to 
mount  them  on  the  insulating  rings  and  permanent  clamps.  As  seg- 
ments are  held  only  at  the  finished  surfaces,  it  is  necessary  that  each 


r 


\N 


4-- 


9/ 


<•>*, 


•-n 


b-:'-L 


262  ELECTRICAL   DESIGNS 

make  solid  contact  with  all  of  the  mica  rings,  as  the  side  pressure  of  the 
other  segments  is  not  sufficient  to  prevent  motion  either  up  or  down. 
The  double  ring  clamps  are  of  special  value  when  the  segments  are  to 
be  brought  to  a  firm  bearing  on  the  mica  rings,  since,  when  necessary, 
the  set  screws  can  be  let  up  a  little  in  order  to  allow  the  segments  to 
slide  over  the  horizontal  mica  rings,  and  then  set  up  until  each  segment 
beds  firmly  on  the  mica.  The  permanent  clamps  once  in  place,  the  out- 
side rings  are  removed  and  the  surface  of  the  segments  finished  up,  first 
by  turning  with  a  diamond-pointed  tool  and  later  with  a  single  cut  file 
or  the  finest  grade  of  sand  paper. 

It  may  be  well  to  add  the  oft  repeated  warning  that  emery  cloth 
should  never  be  used  on  a  commutator,  as  the  emery  sticks  in  the  copper. 

The  above  simple  and  inexpensive  tools  will  multiply  several  times 
the  amount  of  commutator  work  a  man  can  turn  out  daily  with  the  de- 
vices now  common  in  some  shops  devoted  to  the  repair  and  even  the 
manufacture  of  electrical  machinery. 


ALGEBRA   MADE    EASY. 

BY 

EDWIN  J.   HOUSTON,   PH.D.  and  A.  E.  KENNELLY,  Sc.D. 


CONTENTS. 

CHAPTEU  I.— Introduction.  II.— The  Symbols  Commonly  employed  in  Algebra  with  Their  Meanings. 
III. — Powers  and  Roots.  IV. — Radicals.  V. — Logarithms.  VI. — Trigonometry.  VII. — Differen- 
tial Calculus.  VIII.— Integral  Calculus. 


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The  book  is  specially  designed  for  the  beginner,  and  for  all  who  have  not  been 
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THE  INTERPRETATION 

OF 

MATHEMATICAL  FORMULAE 

BY 

EDWIN  J.  HOUSTON,  PH.D.  and  A.  E.  KENNELLY,  Sc.D. 


CONTENTS. 

CHAPTER  I.— Addition.  II.— Substraction .  III.— Multiplication.  IV.— Division.  V.— Involution. 
Powers.  VI.— Evolution.  Roots.  VII.— Equations.  VIII.— Logarithms.  IX.— Trigonometry. 
X. — Hyperbolic  Trigonometrical  Functions.  XI. — Differential  Calculus.  XII. — Integral  Calcu- 
lus. XIII.— Determinants.  XIV.— Synopsis  of  Symbols. 


Cloth.    225  Pages,  <J  Diagrams.    Price,  $1.25. 


A  new  work  for  students'  use  and  to  aid  all  who  are  educating  themselves  on  the 
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class  of  formulas  used  in  electrical  calculations.  A  study  of  this  little  book  will  enable 
any  engineer  to  read  understandingly  the  mathematical  expressions  found  in  technical 
books  and  periodicals. 

AflERICAN    ELECTRICIAN    COMPANY, 

BEARD    BUILDING,    NEW    YORK. 


THE   POCKET 

ELECTRICAL  DICTIONARY 

BY 

EDWIN  J.   HOUSTON,   A.  M.,   PH.D.   (Princeton). 


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peared in  any  other  single  book. 

It  is  published  in  a  convenient  form  for  those  who  only  seek  brief  definitions.  The 
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ELECTRICITY 

ONE    HUNDRED    YEARS    AGO    AND    TO=DAY, 

BY  EDWIN  J.  HOUSTON,  PH.D.  (Princeton). 


Cloth.  199  pages,  illustrated.    Price,  $1.OO. 


In  tracing  the  history  of  electrical  science  from  practically  its  birth  to  the  present 
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a  result  of  these  researches  several  revisions  as  to  the  date  of  discovery  of  some  im- 
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obtained  from  the  book,  as  the  author  is  not  satisfied  to  merely  give  the  history  of  a 
discovery,  but  also  adds  a  concise  and  clear  explanation  of  it. 

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Practical  Features  of  Telephone  Work 

BY   A.    H.    DOBBS. 


CONTENTS. 

Pitfalls  in  Starting.— Poor  versus  Good  "Work.— Starting  a  New  Exchange.— Wire.— Aluminum  Conduc- 
tors.—Weatherproof  Wire.— Country  and  Toll  Lines.— Exchange  Lines  and  Circuits.— Size  of  the 
Return  Wire.— Locating  Lines  and  Poles.— Poles.— Insulators,  Guys,  Bases,  Etc.—  Cross  Connec- 
tion.—Terminal  Poles.— Tree  Trimming.— Cables.— Underground  Conduits.— Manholes.— Elec- 
trolysis.—Fuses  and  Lightning  Arresters.— Selection  of  Instruments.— Transmitter.— Induction 
Coil.—  Receiver.— Wiring.— Instrument  and  Line  Troubles.— Switchboards.— Batteries.— Cross 
Connecting  Boards.— Exchange  Management.— Wire  Tables  and  Formulas.  —  Supporting  Capacity 
of  Galvanized  Strands. 


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The  matter  contained  in  this  book  is  entirely  practical  in  its  bearing,  and  the  result 
of  the  author's  experience  covering  fourteen  years  of  active  work.  All  branches  of 
practical  telephonic  construction  are  treated.  Much  attention  is  given  to  the  pole  line, 
including  its  location,  pole-setting,  stringing  conductors,  transpositions,  cross  connec- 
tions, corner  and  junction  poles,  etc.  Underground  construction  and  cable  work  form 
the  subject  of  several  chapters.  In  the  part  devoted  to  the  exchange,  the  switchboard 
is  treated,  and  a  detailed  description  given  of  the  central  battery  system.  The  book  is 
particularly  adapted  for  those  actually  engaged  in  every  day  telephonic  work,  who, 
from  its  pages,  will  be  enabled  to  derive  much  information  to  assist  them  as  new 
problems  arise. 


EXPERIflENTS    WITH 

ALTERNATING   CURRENTS 

OF   HIGH    POTENTIAL   AND    HIGH    FREQUENCY. 

BY  NIKOLA  TESLA. 


ClotJi.  14=6  pages,  with  Portrait  and  35  Illus.    Price,  $1.OO. 


Since  the  discovery  of  the  telephone  few  researches  in  electricity  have  created  as 
widespread  interest  as  those  of  Nikola  Tcsla  into  alternate  currents  of  high  potential 
and  high  frequency.  The  currents  of  enormously  high  frequency  and  voltage  gener- 
ated by  "Mr.  Tccla  developed  properties  previously  entirely  unsuspected,  and  which 
produced  phenomena  of  startling  character.  The  subject  is  popularly  treated,  and  as 
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Copies  of  this  or  any  other  electrical  book  published  will  be  sent  by  mail,  postage  prc* 
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ELECTRICITY  MADE  EASY 

BY   SIMPLE    LANGUAGE    AND   COPIOUS    ILLUSTRATION 

BY 

EDWIN  J.  HOUSTON,   PH.  D.,   and  A.  B.   KKNNELL,Y,  Sc.  D. 


CONTENTS. 

CHAPTER  T.—  The  Turning  of  an  Electric  Lamp  in  the  House.  IT.— How  the  Electric  Wires  n  re  Distributed 
Through  the  House.  HI.— How  the  Electric  Street  Mains  Supply  the  House.  IV.— How  the  Street 
Mains  are  Supplied  with  Electricity.  V.—  The  Electric  Lighting  Station.  VI.— How  the  Incandescent 
Lamp  Operates.  VII.— How  the  Incandescent  Lamp  is  Made.  VIII.— How  the  Electric  Current  Sup- 
plied to  the  House  is  Measured.  IX.— How  the  Arc  Lamp  Operates.  X.— How  the  Light  of  Electric 
Lamps  is  Best  Distributed.  XI.^The  Voltaic  Cell  and  How  it  Operates.  XII.— The  Electric  Bell 
and  How  it  Operates.  XIII.— The  Electric  Telegraph  and  How  it  Operates.  XIV.— How  the  Dynamo 
Operates.  XV.— How  the  Electric  Motor  Operates.  XVI.— The  Telephone  aud  How  it  Operates. 
XVII. — Some  Other  Applications  of  Electricity. 


Cloth.    348  2>ages,   207  illustrations.    Price,  $1.25. 


The  authors  have  taken  great  pains  to  tell  the  story  of  electricity  in  a  clear,  com- 
prehensive style  so  that  beginners  and  laymen  rannot  fail  to  follow  understandingly  the 
text.  Many  analogies  are  given  which  simplify  what  is  usually  a  difficult  technical 
subject,  and  the  book  is  entirely  devoid  of  mathematics.  Everyday  operations  in  con- 
nection with  electrical  apparatus,  usually  performed  in  a  mechanical  and  wholly 
unknowing  spirit,  are  fully  and  clearly  explained. 

Recent  Types 

of 

Dynamo  Electric  flachinery 

BY 

EDWIN  J.  HOUSTON,  Ph.  D.  and  A.  E.  KKNNKLLY,  Sc.  D. 

Profusely   Illustrated  with   over  600  Magnificent   Engravings   Iv     the   best  known 
Process,  shoivn  in   Color  t  'including   Tables  of  exceptional  value. 


CONTENTS. 

JHAPTER  I. — Introduction.  II. — Direct-Driven  Continuous  Current  Generators  for  Isolated  Plants.  III. — 
Belt-Driven  Continuous  Current  Generators  for  Isolated  Plants.  IV. — Continuous  Current  Central 
Station  Generators.  V.— Central  Station  Arc  Lamp  Generators.  VI.— Some  Miscellaneous  Types  of 
Continuous  Current  Generators.  VII. — Alternating  Current  Generators.  VII f. — Multiphase  Alterna- 
tors. IX. — Alternating  Current  Transformers.  X. — Continuous  Current  Motors.  XI. — Locomotors. 
XII. — Alternating  Current  Motors.  XIII. — Regulators  for  Alternating  Currents  Circuits.  XIV. — 
Secondary  Generators. 


Cloth.    612  pages,    4=35  illustrations.    Price,  $4.00. 


Although  many  books  have  been  written  on  the  subject  of  dynamo-electric  machin- 
ery, yet,  so  far  as  the  authors  are  aware,  none  have  yet  appeared  that  have  been 
devoted  entirely  to  American  tvpes  of  machines.  The  book  is  not  a  treatise  concerning 
the  principles  of  dynamo-electric  machinery,  or  the  theory  of  its  operation,  but  a 
description  treatise  of  the  various  types  of  machines  made  by  different  manufacturers, 
with  their  sizes,  data,  functions  and  capabilities. 


AHERICAN    ELECTRICIAN    COMPANY, 

BEARD   BUILDING,    NEW   YORK. 


Electrical    Engineering    Leaflets 


BY 


BY  EDWIN  J.  HOUSTON,  Pn.D.  and  A.  E.  KENNELLY,  ScD. 
In   Three  Grades. 


Elementary  Grade.  296  Pages,  121  Illus.  Price,  $1.5O. 
I  intermediate  Grade.  3OO  Pages,  14O  Illus.  Price,  $1.5O. 
Advanced  Grade.  296  Pages,  121  Illus.  Price,  $1.5O. 


This  series  has  been  prepared  for  the  purpose  of  presenting,  con- 
cisely and  accurately,  the  fundamental  principles  of  electrical  science  as 
applied  in  practical  work.  Each  of  the  three  grades  is  complete  in  itself, 
though  one  may  be  used  as  a  stepping  stone  to  the  next  higher  grade. 
The  Elementary  Grade  is  intended  for  those  electrical  artisans,  linemen, 
motormen,  central  station  operators  or  electrical  mechanics  generally, 
i^vho  have  had  no  previous  instruction  in  electrical  science.  Here  the 
i  mathematical  treatment  is  limited  to  arithmetic,  and  the  principles  are 
illustrated  by  examples  taken  from  actual  practice.  The  Intermediate 
I  Grade  is  intended  for  those  who  have  mastered  the  first  volume  of  the 
series,  and  for  students  of  electricity  in  high  schools  and  colleges.  This 
volume,  moreover,  contains  such  information  concerning  the  science  o£ 
Electricity  as  should  be  acquired  by  those  desiring  general  mental  culture. 
The  Advanced  Grade  is  designed  for  readers  with  some  mathematical 
preparation,  and  for  students  taking  an  electrical  engineering  course  in 
Colleges  or  universities. 

Copies  of  this  or  any  other  electrical  book  published  will  be  sent  by  mai?t 
postage  prepaid,  to  any  address  in  the  world,  on  receipt  of  price. 

AMERICAN    ELECTRICIAN    COHPANY, 

BEARD    BUILDING,  NEW  YORK. 


Elementary  Electro=Technical  Series. 

BY 

EDWIN].  HOUSTON,  PH.D.,  and  A.  E.  KENNELLY,  Sc.D. 

Alternating  Electric  Currents.  Electric  Incandescent  Lighting. 

Electric  Heating.  Electric  Motor. 

Electromagnetism.  Electric  Street  Railways. 

Electricity  in  Electro-Therapeutics.  Electric  Telephony. 

Electric  Arc  Lighting.  ELctric  Telegraphy, 


Cloth.    Price  per  Volume,  $1.0O. 


The  publication  of  this  series  of  elementary  electro-technical  treatises 
on  applied  electricity  has  been  undertaken  to  meet  a  demand  which  is 
believed  to  exist  on  the  part  of  the  public  and  others  for  reliable  informa- 
-  tion  regarding  such  matters  in  electricity  as  cannot  be  readily  understood 
by  those  not  specially  trained  in  electro-technics.  The  general  public, 
students  of  elementary  electricity  and  the  many  interested  in  the  subject 
from  a  financial  or  other  indirect  connection,  as  well  as  electricians  desir- 
ing information  in  other  branches  than  their  own,  will  find  in  these  works 
precise  and  authoritative  statements  concerning  the  several  branches  of 
applied  electrical  science  of  which  the  separate  volumes  treat.  The  repu- 
tation of  the  authors  and  their  recognized  abilities  as  writers,  are  a 
sufficient  guarantee  for  the  accuracy  and  reliability  of  the  statements  con- 
tained. The  entire  issue,  though  published  in  a  series  of  ten  volumes,  is 
nevertheless  so  prepared  that  each  book  is  complete  in  itself  and  can  be 
understood  independently  of  the  others.  The  volumes  are  profusely  illus- 
trated, printed  on  a  superior  quality  of  paper,  and  handsomely  bound  in 
covers  of  a  special  design. 

Copies  of  this  or  any  other  electrical  book  published  will  be   sent  by 
postage  prepaid,  to  any  address  in  the  world,  on  receipt  of  price.          , 

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AMERICAN 

ELECTRICIAN 

A  Journal  of  Practical  Electrical  and  Mechanical  Engineering. 


THE    LARGEST    PAID    CIRCULATION    OF   ANY 
ELECTRICAL    JOURNAL    IN    THE    WORLD.   .   . 


A  PRACTICAL   PAPER   FOR,  PRACTICAL  MEN. 


ITS  POLICY  consists  in  printing  only  matter  of  in- 
trinsic value,  prepared  by  thoroughly  competent 
writers,  and  presented  without  a  burden  of  theoretical 
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sacrifice  in  accuracy  or  thoroughness.  It  has  solved  the 
problem  of  a  practical  journal  appealing  alike  to  the  pro- 
fessional graduate  and  to  those  who  have  not  had  the 
advantage  of  a  technical  education. 

Among  its  features  are  descriptions  of  Central 
Station,  Electric  Railway  and  Transmission  Plants, 
articles  on  Steam  and  Mechanical  Engineering,  Interior 
Wiring,  Telephone  Practice,  Construction  of  Apparatus, 
Electric  Measurements  and  numerous  other  subjects  of 
direct  practical  interest. 


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THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
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AN  INITIAL  FINE  OF  25  CENTS 

WILL  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY  AND  TO  $1.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


sJA,i   25  1939 

APR    5   1943 

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8Aor'59BP« 

REG'D  LD 

MAR  2  5  1959 

LD  21-95m-7,'37 

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